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+# orm/mapper.py
+# Copyright (C) 2005-2025 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: https://www.opensource.org/licenses/mit-license.php
+# mypy: allow-untyped-defs, allow-untyped-calls
+
+"""Logic to map Python classes to and from selectables.
+
+Defines the :class:`~sqlalchemy.orm.mapper.Mapper` class, the central
+configurational unit which associates a class with a database table.
+
+This is a semi-private module; the main configurational API of the ORM is
+available in :class:`~sqlalchemy.orm.`.
+
+"""
+from __future__ import annotations
+
+from collections import deque
+from functools import reduce
+from itertools import chain
+import sys
+import threading
+from typing import Any
+from typing import Callable
+from typing import cast
+from typing import Collection
+from typing import Deque
+from typing import Dict
+from typing import FrozenSet
+from typing import Generic
+from typing import Iterable
+from typing import Iterator
+from typing import List
+from typing import Mapping
+from typing import Optional
+from typing import Sequence
+from typing import Set
+from typing import Tuple
+from typing import Type
+from typing import TYPE_CHECKING
+from typing import TypeVar
+from typing import Union
+import weakref
+
+from . import attributes
+from . import exc as orm_exc
+from . import instrumentation
+from . import loading
+from . import properties
+from . import util as orm_util
+from ._typing import _O
+from .base import _class_to_mapper
+from .base import _parse_mapper_argument
+from .base import _state_mapper
+from .base import PassiveFlag
+from .base import state_str
+from .interfaces import _MappedAttribute
+from .interfaces import EXT_SKIP
+from .interfaces import InspectionAttr
+from .interfaces import MapperProperty
+from .interfaces import ORMEntityColumnsClauseRole
+from .interfaces import ORMFromClauseRole
+from .interfaces import StrategizedProperty
+from .path_registry import PathRegistry
+from .. import event
+from .. import exc as sa_exc
+from .. import inspection
+from .. import log
+from .. import schema
+from .. import sql
+from .. import util
+from ..event import dispatcher
+from ..event import EventTarget
+from ..sql import base as sql_base
+from ..sql import coercions
+from ..sql import expression
+from ..sql import operators
+from ..sql import roles
+from ..sql import TableClause
+from ..sql import util as sql_util
+from ..sql import visitors
+from ..sql.cache_key import MemoizedHasCacheKey
+from ..sql.elements import KeyedColumnElement
+from ..sql.schema import Column
+from ..sql.schema import Table
+from ..sql.selectable import LABEL_STYLE_TABLENAME_PLUS_COL
+from ..util import HasMemoized
+from ..util import HasMemoized_ro_memoized_attribute
+from ..util.typing import Literal
+
+if TYPE_CHECKING:
+    from ._typing import _IdentityKeyType
+    from ._typing import _InstanceDict
+    from ._typing import _ORMColumnExprArgument
+    from ._typing import _RegistryType
+    from .decl_api import registry
+    from .dependency import DependencyProcessor
+    from .descriptor_props import CompositeProperty
+    from .descriptor_props import SynonymProperty
+    from .events import MapperEvents
+    from .instrumentation import ClassManager
+    from .path_registry import CachingEntityRegistry
+    from .properties import ColumnProperty
+    from .relationships import RelationshipProperty
+    from .state import InstanceState
+    from .util import ORMAdapter
+    from ..engine import Row
+    from ..engine import RowMapping
+    from ..sql._typing import _ColumnExpressionArgument
+    from ..sql._typing import _EquivalentColumnMap
+    from ..sql.base import ReadOnlyColumnCollection
+    from ..sql.elements import ColumnClause
+    from ..sql.elements import ColumnElement
+    from ..sql.selectable import FromClause
+    from ..util import OrderedSet
+
+
+_T = TypeVar("_T", bound=Any)
+_MP = TypeVar("_MP", bound="MapperProperty[Any]")
+_Fn = TypeVar("_Fn", bound="Callable[..., Any]")
+
+
+_WithPolymorphicArg = Union[
+    Literal["*"],
+    Tuple[
+        Union[Literal["*"], Sequence[Union["Mapper[Any]", Type[Any]]]],
+        Optional["FromClause"],
+    ],
+    Sequence[Union["Mapper[Any]", Type[Any]]],
+]
+
+
+_mapper_registries: weakref.WeakKeyDictionary[_RegistryType, bool] = (
+    weakref.WeakKeyDictionary()
+)
+
+
+def _all_registries() -> Set[registry]:
+    with _CONFIGURE_MUTEX:
+        return set(_mapper_registries)
+
+
+def _unconfigured_mappers() -> Iterator[Mapper[Any]]:
+    for reg in _all_registries():
+        yield from reg._mappers_to_configure()
+
+
+_already_compiling = False
+
+
+# a constant returned by _get_attr_by_column to indicate
+# this mapper is not handling an attribute for a particular
+# column
+NO_ATTRIBUTE = util.symbol("NO_ATTRIBUTE")
+
+# lock used to synchronize the "mapper configure" step
+_CONFIGURE_MUTEX = threading.RLock()
+
+
+@inspection._self_inspects
+@log.class_logger
+class Mapper(
+    ORMFromClauseRole,
+    ORMEntityColumnsClauseRole[_O],
+    MemoizedHasCacheKey,
+    InspectionAttr,
+    log.Identified,
+    inspection.Inspectable["Mapper[_O]"],
+    EventTarget,
+    Generic[_O],
+):
+    """Defines an association between a Python class and a database table or
+    other relational structure, so that ORM operations against the class may
+    proceed.
+
+    The :class:`_orm.Mapper` object is instantiated using mapping methods
+    present on the :class:`_orm.registry` object.  For information
+    about instantiating new :class:`_orm.Mapper` objects, see
+    :ref:`orm_mapping_classes_toplevel`.
+
+    """
+
+    dispatch: dispatcher[Mapper[_O]]
+
+    _dispose_called = False
+    _configure_failed: Any = False
+    _ready_for_configure = False
+
+    @util.deprecated_params(
+        non_primary=(
+            "1.3",
+            "The :paramref:`.mapper.non_primary` parameter is deprecated, "
+            "and will be removed in a future release.  The functionality "
+            "of non primary mappers is now better suited using the "
+            ":class:`.AliasedClass` construct, which can also be used "
+            "as the target of a :func:`_orm.relationship` in 1.3.",
+        ),
+    )
+    def __init__(
+        self,
+        class_: Type[_O],
+        local_table: Optional[FromClause] = None,
+        properties: Optional[Mapping[str, MapperProperty[Any]]] = None,
+        primary_key: Optional[Iterable[_ORMColumnExprArgument[Any]]] = None,
+        non_primary: bool = False,
+        inherits: Optional[Union[Mapper[Any], Type[Any]]] = None,
+        inherit_condition: Optional[_ColumnExpressionArgument[bool]] = None,
+        inherit_foreign_keys: Optional[
+            Sequence[_ORMColumnExprArgument[Any]]
+        ] = None,
+        always_refresh: bool = False,
+        version_id_col: Optional[_ORMColumnExprArgument[Any]] = None,
+        version_id_generator: Optional[
+            Union[Literal[False], Callable[[Any], Any]]
+        ] = None,
+        polymorphic_on: Optional[
+            Union[_ORMColumnExprArgument[Any], str, MapperProperty[Any]]
+        ] = None,
+        _polymorphic_map: Optional[Dict[Any, Mapper[Any]]] = None,
+        polymorphic_identity: Optional[Any] = None,
+        concrete: bool = False,
+        with_polymorphic: Optional[_WithPolymorphicArg] = None,
+        polymorphic_abstract: bool = False,
+        polymorphic_load: Optional[Literal["selectin", "inline"]] = None,
+        allow_partial_pks: bool = True,
+        batch: bool = True,
+        column_prefix: Optional[str] = None,
+        include_properties: Optional[Sequence[str]] = None,
+        exclude_properties: Optional[Sequence[str]] = None,
+        passive_updates: bool = True,
+        passive_deletes: bool = False,
+        confirm_deleted_rows: bool = True,
+        eager_defaults: Literal[True, False, "auto"] = "auto",
+        legacy_is_orphan: bool = False,
+        _compiled_cache_size: int = 100,
+    ):
+        r"""Direct constructor for a new :class:`_orm.Mapper` object.
+
+        The :class:`_orm.Mapper` constructor is not called directly, and
+        is normally invoked through the
+        use of the :class:`_orm.registry` object through either the
+        :ref:`Declarative <orm_declarative_mapping>` or
+        :ref:`Imperative <orm_imperative_mapping>` mapping styles.
+
+        .. versionchanged:: 2.0 The public facing ``mapper()`` function is
+           removed; for a classical mapping configuration, use the
+           :meth:`_orm.registry.map_imperatively` method.
+
+        Parameters documented below may be passed to either the
+        :meth:`_orm.registry.map_imperatively` method, or may be passed in the
+        ``__mapper_args__`` declarative class attribute described at
+        :ref:`orm_declarative_mapper_options`.
+
+        :param class\_: The class to be mapped.  When using Declarative,
+          this argument is automatically passed as the declared class
+          itself.
+
+        :param local_table: The :class:`_schema.Table` or other
+           :class:`_sql.FromClause` (i.e. selectable) to which the class is
+           mapped. May be ``None`` if this mapper inherits from another mapper
+           using single-table inheritance. When using Declarative, this
+           argument is automatically passed by the extension, based on what is
+           configured via the :attr:`_orm.DeclarativeBase.__table__` attribute
+           or via the :class:`_schema.Table` produced as a result of
+           the :attr:`_orm.DeclarativeBase.__tablename__` attribute being
+           present.
+
+        :param polymorphic_abstract: Indicates this class will be mapped in a
+            polymorphic hierarchy, but not directly instantiated. The class is
+            mapped normally, except that it has no requirement for a
+            :paramref:`_orm.Mapper.polymorphic_identity` within an inheritance
+            hierarchy. The class however must be part of a polymorphic
+            inheritance scheme which uses
+            :paramref:`_orm.Mapper.polymorphic_on` at the base.
+
+            .. versionadded:: 2.0
+
+            .. seealso::
+
+                :ref:`orm_inheritance_abstract_poly`
+
+        :param always_refresh: If True, all query operations for this mapped
+           class will overwrite all data within object instances that already
+           exist within the session, erasing any in-memory changes with
+           whatever information was loaded from the database. Usage of this
+           flag is highly discouraged; as an alternative, see the method
+           :meth:`_query.Query.populate_existing`.
+
+        :param allow_partial_pks: Defaults to True.  Indicates that a
+           composite primary key with some NULL values should be considered as
+           possibly existing within the database. This affects whether a
+           mapper will assign an incoming row to an existing identity, as well
+           as if :meth:`.Session.merge` will check the database first for a
+           particular primary key value. A "partial primary key" can occur if
+           one has mapped to an OUTER JOIN, for example.
+
+           The :paramref:`.orm.Mapper.allow_partial_pks` parameter also
+           indicates to the ORM relationship lazy loader, when loading a
+           many-to-one related object, if a composite primary key that has
+           partial NULL values should result in an attempt to load from the
+           database, or if a load attempt is not necessary.
+
+           .. versionadded:: 2.0.36 :paramref:`.orm.Mapper.allow_partial_pks`
+              is consulted by the relationship lazy loader strategy, such that
+              when set to False, a SELECT for a composite primary key that
+              has partial NULL values will not be emitted.
+
+        :param batch: Defaults to ``True``, indicating that save operations
+           of multiple entities can be batched together for efficiency.
+           Setting to False indicates
+           that an instance will be fully saved before saving the next
+           instance.  This is used in the extremely rare case that a
+           :class:`.MapperEvents` listener requires being called
+           in between individual row persistence operations.
+
+        :param column_prefix: A string which will be prepended
+           to the mapped attribute name when :class:`_schema.Column`
+           objects are automatically assigned as attributes to the
+           mapped class.  Does not affect :class:`.Column` objects that
+           are mapped explicitly in the :paramref:`.Mapper.properties`
+           dictionary.
+
+           This parameter is typically useful with imperative mappings
+           that keep the :class:`.Table` object separate.  Below, assuming
+           the ``user_table`` :class:`.Table` object has columns named
+           ``user_id``, ``user_name``, and ``password``::
+
+                class User(Base):
+                    __table__ = user_table
+                    __mapper_args__ = {"column_prefix": "_"}
+
+           The above mapping will assign the ``user_id``, ``user_name``, and
+           ``password`` columns to attributes named ``_user_id``,
+           ``_user_name``, and ``_password`` on the mapped ``User`` class.
+
+           The :paramref:`.Mapper.column_prefix` parameter is uncommon in
+           modern use. For dealing with reflected tables, a more flexible
+           approach to automating a naming scheme is to intercept the
+           :class:`.Column` objects as they are reflected; see the section
+           :ref:`mapper_automated_reflection_schemes` for notes on this usage
+           pattern.
+
+        :param concrete: If True, indicates this mapper should use concrete
+           table inheritance with its parent mapper.
+
+           See the section :ref:`concrete_inheritance` for an example.
+
+        :param confirm_deleted_rows: defaults to True; when a DELETE occurs
+          of one more rows based on specific primary keys, a warning is
+          emitted when the number of rows matched does not equal the number
+          of rows expected.  This parameter may be set to False to handle the
+          case where database ON DELETE CASCADE rules may be deleting some of
+          those rows automatically.  The warning may be changed to an
+          exception in a future release.
+
+        :param eager_defaults: if True, the ORM will immediately fetch the
+          value of server-generated default values after an INSERT or UPDATE,
+          rather than leaving them as expired to be fetched on next access.
+          This can be used for event schemes where the server-generated values
+          are needed immediately before the flush completes.
+
+          The fetch of values occurs either by using ``RETURNING`` inline
+          with the ``INSERT`` or ``UPDATE`` statement, or by adding an
+          additional ``SELECT`` statement subsequent to the ``INSERT`` or
+          ``UPDATE``, if the backend does not support ``RETURNING``.
+
+          The use of ``RETURNING`` is extremely performant in particular for
+          ``INSERT`` statements where SQLAlchemy can take advantage of
+          :ref:`insertmanyvalues <engine_insertmanyvalues>`, whereas the use of
+          an additional ``SELECT`` is relatively poor performing, adding
+          additional SQL round trips which would be unnecessary if these new
+          attributes are not to be accessed in any case.
+
+          For this reason, :paramref:`.Mapper.eager_defaults` defaults to the
+          string value ``"auto"``, which indicates that server defaults for
+          INSERT should be fetched using ``RETURNING`` if the backing database
+          supports it and if the dialect in use supports "insertmanyreturning"
+          for an INSERT statement. If the backing database does not support
+          ``RETURNING`` or "insertmanyreturning" is not available, server
+          defaults will not be fetched.
+
+          .. versionchanged:: 2.0.0rc1 added the "auto" option for
+             :paramref:`.Mapper.eager_defaults`
+
+          .. seealso::
+
+                :ref:`orm_server_defaults`
+
+          .. versionchanged:: 2.0.0  RETURNING now works with multiple rows
+             INSERTed at once using the
+             :ref:`insertmanyvalues <engine_insertmanyvalues>` feature, which
+             among other things allows the :paramref:`.Mapper.eager_defaults`
+             feature to be very performant on supporting backends.
+
+        :param exclude_properties: A list or set of string column names to
+          be excluded from mapping.
+
+          .. seealso::
+
+            :ref:`include_exclude_cols`
+
+        :param include_properties: An inclusive list or set of string column
+          names to map.
+
+          .. seealso::
+
+            :ref:`include_exclude_cols`
+
+        :param inherits: A mapped class or the corresponding
+          :class:`_orm.Mapper`
+          of one indicating a superclass to which this :class:`_orm.Mapper`
+          should *inherit* from.   The mapped class here must be a subclass
+          of the other mapper's class.   When using Declarative, this argument
+          is passed automatically as a result of the natural class
+          hierarchy of the declared classes.
+
+          .. seealso::
+
+            :ref:`inheritance_toplevel`
+
+        :param inherit_condition: For joined table inheritance, a SQL
+           expression which will
+           define how the two tables are joined; defaults to a natural join
+           between the two tables.
+
+        :param inherit_foreign_keys: When ``inherit_condition`` is used and
+           the columns present are missing a :class:`_schema.ForeignKey`
+           configuration, this parameter can be used to specify which columns
+           are "foreign".  In most cases can be left as ``None``.
+
+        :param legacy_is_orphan: Boolean, defaults to ``False``.
+          When ``True``, specifies that "legacy" orphan consideration
+          is to be applied to objects mapped by this mapper, which means
+          that a pending (that is, not persistent) object is auto-expunged
+          from an owning :class:`.Session` only when it is de-associated
+          from *all* parents that specify a ``delete-orphan`` cascade towards
+          this mapper.  The new default behavior is that the object is
+          auto-expunged when it is de-associated with *any* of its parents
+          that specify ``delete-orphan`` cascade.  This behavior is more
+          consistent with that of a persistent object, and allows behavior to
+          be consistent in more scenarios independently of whether or not an
+          orphan object has been flushed yet or not.
+
+          See the change note and example at :ref:`legacy_is_orphan_addition`
+          for more detail on this change.
+
+        :param non_primary: Specify that this :class:`_orm.Mapper`
+          is in addition
+          to the "primary" mapper, that is, the one used for persistence.
+          The :class:`_orm.Mapper` created here may be used for ad-hoc
+          mapping of the class to an alternate selectable, for loading
+          only.
+
+          .. seealso::
+
+            :ref:`relationship_aliased_class` - the new pattern that removes
+            the need for the :paramref:`_orm.Mapper.non_primary` flag.
+
+        :param passive_deletes: Indicates DELETE behavior of foreign key
+           columns when a joined-table inheritance entity is being deleted.
+           Defaults to ``False`` for a base mapper; for an inheriting mapper,
+           defaults to ``False`` unless the value is set to ``True``
+           on the superclass mapper.
+
+           When ``True``, it is assumed that ON DELETE CASCADE is configured
+           on the foreign key relationships that link this mapper's table
+           to its superclass table, so that when the unit of work attempts
+           to delete the entity, it need only emit a DELETE statement for the
+           superclass table, and not this table.
+
+           When ``False``, a DELETE statement is emitted for this mapper's
+           table individually.  If the primary key attributes local to this
+           table are unloaded, then a SELECT must be emitted in order to
+           validate these attributes; note that the primary key columns
+           of a joined-table subclass are not part of the "primary key" of
+           the object as a whole.
+
+           Note that a value of ``True`` is **always** forced onto the
+           subclass mappers; that is, it's not possible for a superclass
+           to specify passive_deletes without this taking effect for
+           all subclass mappers.
+
+           .. seealso::
+
+               :ref:`passive_deletes` - description of similar feature as
+               used with :func:`_orm.relationship`
+
+               :paramref:`.mapper.passive_updates` - supporting ON UPDATE
+               CASCADE for joined-table inheritance mappers
+
+        :param passive_updates: Indicates UPDATE behavior of foreign key
+           columns when a primary key column changes on a joined-table
+           inheritance mapping.   Defaults to ``True``.
+
+           When True, it is assumed that ON UPDATE CASCADE is configured on
+           the foreign key in the database, and that the database will handle
+           propagation of an UPDATE from a source column to dependent columns
+           on joined-table rows.
+
+           When False, it is assumed that the database does not enforce
+           referential integrity and will not be issuing its own CASCADE
+           operation for an update.  The unit of work process will
+           emit an UPDATE statement for the dependent columns during a
+           primary key change.
+
+           .. seealso::
+
+               :ref:`passive_updates` - description of a similar feature as
+               used with :func:`_orm.relationship`
+
+               :paramref:`.mapper.passive_deletes` - supporting ON DELETE
+               CASCADE for joined-table inheritance mappers
+
+        :param polymorphic_load: Specifies "polymorphic loading" behavior
+         for a subclass in an inheritance hierarchy (joined and single
+         table inheritance only).   Valid values are:
+
+          * "'inline'" - specifies this class should be part of
+            the "with_polymorphic" mappers, e.g. its columns will be included
+            in a SELECT query against the base.
+
+          * "'selectin'" - specifies that when instances of this class
+            are loaded, an additional SELECT will be emitted to retrieve
+            the columns specific to this subclass.  The SELECT uses
+            IN to fetch multiple subclasses at once.
+
+         .. versionadded:: 1.2
+
+         .. seealso::
+
+            :ref:`with_polymorphic_mapper_config`
+
+            :ref:`polymorphic_selectin`
+
+        :param polymorphic_on: Specifies the column, attribute, or
+          SQL expression used to determine the target class for an
+          incoming row, when inheriting classes are present.
+
+          May be specified as a string attribute name, or as a SQL
+          expression such as a :class:`_schema.Column` or in a Declarative
+          mapping a :func:`_orm.mapped_column` object.  It is typically
+          expected that the SQL expression corresponds to a column in the
+          base-most mapped :class:`.Table`::
+
+            class Employee(Base):
+                __tablename__ = "employee"
+
+                id: Mapped[int] = mapped_column(primary_key=True)
+                discriminator: Mapped[str] = mapped_column(String(50))
+
+                __mapper_args__ = {
+                    "polymorphic_on": discriminator,
+                    "polymorphic_identity": "employee",
+                }
+
+          It may also be specified
+          as a SQL expression, as in this example where we
+          use the :func:`.case` construct to provide a conditional
+          approach::
+
+            class Employee(Base):
+                __tablename__ = "employee"
+
+                id: Mapped[int] = mapped_column(primary_key=True)
+                discriminator: Mapped[str] = mapped_column(String(50))
+
+                __mapper_args__ = {
+                    "polymorphic_on": case(
+                        (discriminator == "EN", "engineer"),
+                        (discriminator == "MA", "manager"),
+                        else_="employee",
+                    ),
+                    "polymorphic_identity": "employee",
+                }
+
+          It may also refer to any attribute using its string name,
+          which is of particular use when using annotated column
+          configurations::
+
+                class Employee(Base):
+                    __tablename__ = "employee"
+
+                    id: Mapped[int] = mapped_column(primary_key=True)
+                    discriminator: Mapped[str]
+
+                    __mapper_args__ = {
+                        "polymorphic_on": "discriminator",
+                        "polymorphic_identity": "employee",
+                    }
+
+          When setting ``polymorphic_on`` to reference an
+          attribute or expression that's not present in the
+          locally mapped :class:`_schema.Table`, yet the value
+          of the discriminator should be persisted to the database,
+          the value of the
+          discriminator is not automatically set on new
+          instances; this must be handled by the user,
+          either through manual means or via event listeners.
+          A typical approach to establishing such a listener
+          looks like::
+
+                from sqlalchemy import event
+                from sqlalchemy.orm import object_mapper
+
+
+                @event.listens_for(Employee, "init", propagate=True)
+                def set_identity(instance, *arg, **kw):
+                    mapper = object_mapper(instance)
+                    instance.discriminator = mapper.polymorphic_identity
+
+          Where above, we assign the value of ``polymorphic_identity``
+          for the mapped class to the ``discriminator`` attribute,
+          thus persisting the value to the ``discriminator`` column
+          in the database.
+
+          .. warning::
+
+             Currently, **only one discriminator column may be set**, typically
+             on the base-most class in the hierarchy. "Cascading" polymorphic
+             columns are not yet supported.
+
+          .. seealso::
+
+            :ref:`inheritance_toplevel`
+
+        :param polymorphic_identity: Specifies the value which
+          identifies this particular class as returned by the column expression
+          referred to by the :paramref:`_orm.Mapper.polymorphic_on` setting. As
+          rows are received, the value corresponding to the
+          :paramref:`_orm.Mapper.polymorphic_on` column expression is compared
+          to this value, indicating which subclass should be used for the newly
+          reconstructed object.
+
+          .. seealso::
+
+            :ref:`inheritance_toplevel`
+
+        :param properties: A dictionary mapping the string names of object
+           attributes to :class:`.MapperProperty` instances, which define the
+           persistence behavior of that attribute.  Note that
+           :class:`_schema.Column`
+           objects present in
+           the mapped :class:`_schema.Table` are automatically placed into
+           ``ColumnProperty`` instances upon mapping, unless overridden.
+           When using Declarative, this argument is passed automatically,
+           based on all those :class:`.MapperProperty` instances declared
+           in the declared class body.
+
+           .. seealso::
+
+               :ref:`orm_mapping_properties` - in the
+               :ref:`orm_mapping_classes_toplevel`
+
+        :param primary_key: A list of :class:`_schema.Column`
+           objects, or alternatively string names of attribute names which
+           refer to :class:`_schema.Column`, which define
+           the primary key to be used against this mapper's selectable unit.
+           This is normally simply the primary key of the ``local_table``, but
+           can be overridden here.
+
+           .. versionchanged:: 2.0.2 :paramref:`_orm.Mapper.primary_key`
+              arguments may be indicated as string attribute names as well.
+
+           .. seealso::
+
+                :ref:`mapper_primary_key` - background and example use
+
+        :param version_id_col: A :class:`_schema.Column`
+           that will be used to keep a running version id of rows
+           in the table.  This is used to detect concurrent updates or
+           the presence of stale data in a flush.  The methodology is to
+           detect if an UPDATE statement does not match the last known
+           version id, a
+           :class:`~sqlalchemy.orm.exc.StaleDataError` exception is
+           thrown.
+           By default, the column must be of :class:`.Integer` type,
+           unless ``version_id_generator`` specifies an alternative version
+           generator.
+
+           .. seealso::
+
+              :ref:`mapper_version_counter` - discussion of version counting
+              and rationale.
+
+        :param version_id_generator: Define how new version ids should
+          be generated.  Defaults to ``None``, which indicates that
+          a simple integer counting scheme be employed.  To provide a custom
+          versioning scheme, provide a callable function of the form::
+
+              def generate_version(version):
+                  return next_version
+
+          Alternatively, server-side versioning functions such as triggers,
+          or programmatic versioning schemes outside of the version id
+          generator may be used, by specifying the value ``False``.
+          Please see :ref:`server_side_version_counter` for a discussion
+          of important points when using this option.
+
+          .. seealso::
+
+             :ref:`custom_version_counter`
+
+             :ref:`server_side_version_counter`
+
+
+        :param with_polymorphic: A tuple in the form ``(<classes>,
+            <selectable>)`` indicating the default style of "polymorphic"
+            loading, that is, which tables are queried at once. <classes> is
+            any single or list of mappers and/or classes indicating the
+            inherited classes that should be loaded at once. The special value
+            ``'*'`` may be used to indicate all descending classes should be
+            loaded immediately. The second tuple argument <selectable>
+            indicates a selectable that will be used to query for multiple
+            classes.
+
+            The :paramref:`_orm.Mapper.polymorphic_load` parameter may be
+            preferable over the use of :paramref:`_orm.Mapper.with_polymorphic`
+            in modern mappings to indicate a per-subclass technique of
+            indicating polymorphic loading styles.
+
+            .. seealso::
+
+                :ref:`with_polymorphic_mapper_config`
+
+        """
+        self.class_ = util.assert_arg_type(class_, type, "class_")
+        self._sort_key = "%s.%s" % (
+            self.class_.__module__,
+            self.class_.__name__,
+        )
+
+        self._primary_key_argument = util.to_list(primary_key)
+        self.non_primary = non_primary
+
+        self.always_refresh = always_refresh
+
+        if isinstance(version_id_col, MapperProperty):
+            self.version_id_prop = version_id_col
+            self.version_id_col = None
+        else:
+            self.version_id_col = (
+                coercions.expect(
+                    roles.ColumnArgumentOrKeyRole,
+                    version_id_col,
+                    argname="version_id_col",
+                )
+                if version_id_col is not None
+                else None
+            )
+
+        if version_id_generator is False:
+            self.version_id_generator = False
+        elif version_id_generator is None:
+            self.version_id_generator = lambda x: (x or 0) + 1
+        else:
+            self.version_id_generator = version_id_generator
+
+        self.concrete = concrete
+        self.single = False
+
+        if inherits is not None:
+            self.inherits = _parse_mapper_argument(inherits)
+        else:
+            self.inherits = None
+
+        if local_table is not None:
+            self.local_table = coercions.expect(
+                roles.StrictFromClauseRole,
+                local_table,
+                disable_inspection=True,
+                argname="local_table",
+            )
+        elif self.inherits:
+            # note this is a new flow as of 2.0 so that
+            # .local_table need not be Optional
+            self.local_table = self.inherits.local_table
+            self.single = True
+        else:
+            raise sa_exc.ArgumentError(
+                f"Mapper[{self.class_.__name__}(None)] has None for a "
+                "primary table argument and does not specify 'inherits'"
+            )
+
+        if inherit_condition is not None:
+            self.inherit_condition = coercions.expect(
+                roles.OnClauseRole, inherit_condition
+            )
+        else:
+            self.inherit_condition = None
+
+        self.inherit_foreign_keys = inherit_foreign_keys
+        self._init_properties = dict(properties) if properties else {}
+        self._delete_orphans = []
+        self.batch = batch
+        self.eager_defaults = eager_defaults
+        self.column_prefix = column_prefix
+
+        # interim - polymorphic_on is further refined in
+        # _configure_polymorphic_setter
+        self.polymorphic_on = (
+            coercions.expect(  # type: ignore
+                roles.ColumnArgumentOrKeyRole,
+                polymorphic_on,
+                argname="polymorphic_on",
+            )
+            if polymorphic_on is not None
+            else None
+        )
+        self.polymorphic_abstract = polymorphic_abstract
+        self._dependency_processors = []
+        self.validators = util.EMPTY_DICT
+        self.passive_updates = passive_updates
+        self.passive_deletes = passive_deletes
+        self.legacy_is_orphan = legacy_is_orphan
+        self._clause_adapter = None
+        self._requires_row_aliasing = False
+        self._inherits_equated_pairs = None
+        self._memoized_values = {}
+        self._compiled_cache_size = _compiled_cache_size
+        self._reconstructor = None
+        self.allow_partial_pks = allow_partial_pks
+
+        if self.inherits and not self.concrete:
+            self.confirm_deleted_rows = False
+        else:
+            self.confirm_deleted_rows = confirm_deleted_rows
+
+        self._set_with_polymorphic(with_polymorphic)
+        self.polymorphic_load = polymorphic_load
+
+        # our 'polymorphic identity', a string name that when located in a
+        #  result set row indicates this Mapper should be used to construct
+        # the object instance for that row.
+        self.polymorphic_identity = polymorphic_identity
+
+        # a dictionary of 'polymorphic identity' names, associating those
+        # names with Mappers that will be used to construct object instances
+        # upon a select operation.
+        if _polymorphic_map is None:
+            self.polymorphic_map = {}
+        else:
+            self.polymorphic_map = _polymorphic_map
+
+        if include_properties is not None:
+            self.include_properties = util.to_set(include_properties)
+        else:
+            self.include_properties = None
+        if exclude_properties:
+            self.exclude_properties = util.to_set(exclude_properties)
+        else:
+            self.exclude_properties = None
+
+        # prevent this mapper from being constructed
+        # while a configure_mappers() is occurring (and defer a
+        # configure_mappers() until construction succeeds)
+        with _CONFIGURE_MUTEX:
+            cast("MapperEvents", self.dispatch._events)._new_mapper_instance(
+                class_, self
+            )
+            self._configure_inheritance()
+            self._configure_class_instrumentation()
+            self._configure_properties()
+            self._configure_polymorphic_setter()
+            self._configure_pks()
+            self.registry._flag_new_mapper(self)
+            self._log("constructed")
+            self._expire_memoizations()
+
+        self.dispatch.after_mapper_constructed(self, self.class_)
+
+    def _prefer_eager_defaults(self, dialect, table):
+        if self.eager_defaults == "auto":
+            if not table.implicit_returning:
+                return False
+
+            return (
+                table in self._server_default_col_keys
+                and dialect.insert_executemany_returning
+            )
+        else:
+            return self.eager_defaults
+
+    def _gen_cache_key(self, anon_map, bindparams):
+        return (self,)
+
+    # ### BEGIN
+    # ATTRIBUTE DECLARATIONS START HERE
+
+    is_mapper = True
+    """Part of the inspection API."""
+
+    represents_outer_join = False
+
+    registry: _RegistryType
+
+    @property
+    def mapper(self) -> Mapper[_O]:
+        """Part of the inspection API.
+
+        Returns self.
+
+        """
+        return self
+
+    @property
+    def entity(self):
+        r"""Part of the inspection API.
+
+        Returns self.class\_.
+
+        """
+        return self.class_
+
+    class_: Type[_O]
+    """The class to which this :class:`_orm.Mapper` is mapped."""
+
+    _identity_class: Type[_O]
+
+    _delete_orphans: List[Tuple[str, Type[Any]]]
+    _dependency_processors: List[DependencyProcessor]
+    _memoized_values: Dict[Any, Callable[[], Any]]
+    _inheriting_mappers: util.WeakSequence[Mapper[Any]]
+    _all_tables: Set[TableClause]
+    _polymorphic_attr_key: Optional[str]
+
+    _pks_by_table: Dict[FromClause, OrderedSet[ColumnClause[Any]]]
+    _cols_by_table: Dict[FromClause, OrderedSet[ColumnElement[Any]]]
+
+    _props: util.OrderedDict[str, MapperProperty[Any]]
+    _init_properties: Dict[str, MapperProperty[Any]]
+
+    _columntoproperty: _ColumnMapping
+
+    _set_polymorphic_identity: Optional[Callable[[InstanceState[_O]], None]]
+    _validate_polymorphic_identity: Optional[
+        Callable[[Mapper[_O], InstanceState[_O], _InstanceDict], None]
+    ]
+
+    tables: Sequence[TableClause]
+    """A sequence containing the collection of :class:`_schema.Table`
+    or :class:`_schema.TableClause` objects which this :class:`_orm.Mapper`
+    is aware of.
+
+    If the mapper is mapped to a :class:`_expression.Join`, or an
+    :class:`_expression.Alias`
+    representing a :class:`_expression.Select`, the individual
+    :class:`_schema.Table`
+    objects that comprise the full construct will be represented here.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    validators: util.immutabledict[str, Tuple[str, Dict[str, Any]]]
+    """An immutable dictionary of attributes which have been decorated
+    using the :func:`_orm.validates` decorator.
+
+    The dictionary contains string attribute names as keys
+    mapped to the actual validation method.
+
+    """
+
+    always_refresh: bool
+    allow_partial_pks: bool
+    version_id_col: Optional[ColumnElement[Any]]
+
+    with_polymorphic: Optional[
+        Tuple[
+            Union[Literal["*"], Sequence[Union[Mapper[Any], Type[Any]]]],
+            Optional[FromClause],
+        ]
+    ]
+
+    version_id_generator: Optional[Union[Literal[False], Callable[[Any], Any]]]
+
+    local_table: FromClause
+    """The immediate :class:`_expression.FromClause` to which this
+    :class:`_orm.Mapper` refers.
+
+    Typically is an instance of :class:`_schema.Table`, may be any
+    :class:`.FromClause`.
+
+    The "local" table is the
+    selectable that the :class:`_orm.Mapper` is directly responsible for
+    managing from an attribute access and flush perspective.   For
+    non-inheriting mappers, :attr:`.Mapper.local_table` will be the same
+    as :attr:`.Mapper.persist_selectable`.  For inheriting mappers,
+    :attr:`.Mapper.local_table` refers to the specific portion of
+    :attr:`.Mapper.persist_selectable` that includes the columns to which
+    this :class:`.Mapper` is loading/persisting, such as a particular
+    :class:`.Table` within a join.
+
+    .. seealso::
+
+        :attr:`_orm.Mapper.persist_selectable`.
+
+        :attr:`_orm.Mapper.selectable`.
+
+    """
+
+    persist_selectable: FromClause
+    """The :class:`_expression.FromClause` to which this :class:`_orm.Mapper`
+    is mapped.
+
+    Typically is an instance of :class:`_schema.Table`, may be any
+    :class:`.FromClause`.
+
+    The :attr:`_orm.Mapper.persist_selectable` is similar to
+    :attr:`.Mapper.local_table`, but represents the :class:`.FromClause` that
+    represents the inheriting class hierarchy overall in an inheritance
+    scenario.
+
+    :attr.`.Mapper.persist_selectable` is also separate from the
+    :attr:`.Mapper.selectable` attribute, the latter of which may be an
+    alternate subquery used for selecting columns.
+    :attr.`.Mapper.persist_selectable` is oriented towards columns that
+    will be written on a persist operation.
+
+    .. seealso::
+
+        :attr:`_orm.Mapper.selectable`.
+
+        :attr:`_orm.Mapper.local_table`.
+
+    """
+
+    inherits: Optional[Mapper[Any]]
+    """References the :class:`_orm.Mapper` which this :class:`_orm.Mapper`
+    inherits from, if any.
+
+    """
+
+    inherit_condition: Optional[ColumnElement[bool]]
+
+    configured: bool = False
+    """Represent ``True`` if this :class:`_orm.Mapper` has been configured.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    .. seealso::
+
+        :func:`.configure_mappers`.
+
+    """
+
+    concrete: bool
+    """Represent ``True`` if this :class:`_orm.Mapper` is a concrete
+    inheritance mapper.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    primary_key: Tuple[Column[Any], ...]
+    """An iterable containing the collection of :class:`_schema.Column`
+    objects
+    which comprise the 'primary key' of the mapped table, from the
+    perspective of this :class:`_orm.Mapper`.
+
+    This list is against the selectable in
+    :attr:`_orm.Mapper.persist_selectable`.
+    In the case of inheriting mappers, some columns may be managed by a
+    superclass mapper.  For example, in the case of a
+    :class:`_expression.Join`, the
+    primary key is determined by all of the primary key columns across all
+    tables referenced by the :class:`_expression.Join`.
+
+    The list is also not necessarily the same as the primary key column
+    collection associated with the underlying tables; the :class:`_orm.Mapper`
+    features a ``primary_key`` argument that can override what the
+    :class:`_orm.Mapper` considers as primary key columns.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    class_manager: ClassManager[_O]
+    """The :class:`.ClassManager` which maintains event listeners
+    and class-bound descriptors for this :class:`_orm.Mapper`.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    single: bool
+    """Represent ``True`` if this :class:`_orm.Mapper` is a single table
+    inheritance mapper.
+
+    :attr:`_orm.Mapper.local_table` will be ``None`` if this flag is set.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    non_primary: bool
+    """Represent ``True`` if this :class:`_orm.Mapper` is a "non-primary"
+    mapper, e.g. a mapper that is used only to select rows but not for
+    persistence management.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    polymorphic_on: Optional[KeyedColumnElement[Any]]
+    """The :class:`_schema.Column` or SQL expression specified as the
+    ``polymorphic_on`` argument
+    for this :class:`_orm.Mapper`, within an inheritance scenario.
+
+    This attribute is normally a :class:`_schema.Column` instance but
+    may also be an expression, such as one derived from
+    :func:`.cast`.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    polymorphic_map: Dict[Any, Mapper[Any]]
+    """A mapping of "polymorphic identity" identifiers mapped to
+    :class:`_orm.Mapper` instances, within an inheritance scenario.
+
+    The identifiers can be of any type which is comparable to the
+    type of column represented by :attr:`_orm.Mapper.polymorphic_on`.
+
+    An inheritance chain of mappers will all reference the same
+    polymorphic map object.  The object is used to correlate incoming
+    result rows to target mappers.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    polymorphic_identity: Optional[Any]
+    """Represent an identifier which is matched against the
+    :attr:`_orm.Mapper.polymorphic_on` column during result row loading.
+
+    Used only with inheritance, this object can be of any type which is
+    comparable to the type of column represented by
+    :attr:`_orm.Mapper.polymorphic_on`.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    base_mapper: Mapper[Any]
+    """The base-most :class:`_orm.Mapper` in an inheritance chain.
+
+    In a non-inheriting scenario, this attribute will always be this
+    :class:`_orm.Mapper`.   In an inheritance scenario, it references
+    the :class:`_orm.Mapper` which is parent to all other :class:`_orm.Mapper`
+    objects in the inheritance chain.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    columns: ReadOnlyColumnCollection[str, Column[Any]]
+    """A collection of :class:`_schema.Column` or other scalar expression
+    objects maintained by this :class:`_orm.Mapper`.
+
+    The collection behaves the same as that of the ``c`` attribute on
+    any :class:`_schema.Table` object,
+    except that only those columns included in
+    this mapping are present, and are keyed based on the attribute name
+    defined in the mapping, not necessarily the ``key`` attribute of the
+    :class:`_schema.Column` itself.   Additionally, scalar expressions mapped
+    by :func:`.column_property` are also present here.
+
+    This is a *read only* attribute determined during mapper construction.
+    Behavior is undefined if directly modified.
+
+    """
+
+    c: ReadOnlyColumnCollection[str, Column[Any]]
+    """A synonym for :attr:`_orm.Mapper.columns`."""
+
+    @util.non_memoized_property
+    @util.deprecated("1.3", "Use .persist_selectable")
+    def mapped_table(self):
+        return self.persist_selectable
+
+    @util.memoized_property
+    def _path_registry(self) -> CachingEntityRegistry:
+        return PathRegistry.per_mapper(self)
+
+    def _configure_inheritance(self):
+        """Configure settings related to inheriting and/or inherited mappers
+        being present."""
+
+        # a set of all mappers which inherit from this one.
+        self._inheriting_mappers = util.WeakSequence()
+
+        if self.inherits:
+            if not issubclass(self.class_, self.inherits.class_):
+                raise sa_exc.ArgumentError(
+                    "Class '%s' does not inherit from '%s'"
+                    % (self.class_.__name__, self.inherits.class_.__name__)
+                )
+
+            self.dispatch._update(self.inherits.dispatch)
+
+            if self.non_primary != self.inherits.non_primary:
+                np = not self.non_primary and "primary" or "non-primary"
+                raise sa_exc.ArgumentError(
+                    "Inheritance of %s mapper for class '%s' is "
+                    "only allowed from a %s mapper"
+                    % (np, self.class_.__name__, np)
+                )
+
+            if self.single:
+                self.persist_selectable = self.inherits.persist_selectable
+            elif self.local_table is not self.inherits.local_table:
+                if self.concrete:
+                    self.persist_selectable = self.local_table
+                    for mapper in self.iterate_to_root():
+                        if mapper.polymorphic_on is not None:
+                            mapper._requires_row_aliasing = True
+                else:
+                    if self.inherit_condition is None:
+                        # figure out inherit condition from our table to the
+                        # immediate table of the inherited mapper, not its
+                        # full table which could pull in other stuff we don't
+                        # want (allows test/inheritance.InheritTest4 to pass)
+                        try:
+                            self.inherit_condition = sql_util.join_condition(
+                                self.inherits.local_table, self.local_table
+                            )
+                        except sa_exc.NoForeignKeysError as nfe:
+                            assert self.inherits.local_table is not None
+                            assert self.local_table is not None
+                            raise sa_exc.NoForeignKeysError(
+                                "Can't determine the inherit condition "
+                                "between inherited table '%s' and "
+                                "inheriting "
+                                "table '%s'; tables have no "
+                                "foreign key relationships established.  "
+                                "Please ensure the inheriting table has "
+                                "a foreign key relationship to the "
+                                "inherited "
+                                "table, or provide an "
+                                "'on clause' using "
+                                "the 'inherit_condition' mapper argument."
+                                % (
+                                    self.inherits.local_table.description,
+                                    self.local_table.description,
+                                )
+                            ) from nfe
+                        except sa_exc.AmbiguousForeignKeysError as afe:
+                            assert self.inherits.local_table is not None
+                            assert self.local_table is not None
+                            raise sa_exc.AmbiguousForeignKeysError(
+                                "Can't determine the inherit condition "
+                                "between inherited table '%s' and "
+                                "inheriting "
+                                "table '%s'; tables have more than one "
+                                "foreign key relationship established.  "
+                                "Please specify the 'on clause' using "
+                                "the 'inherit_condition' mapper argument."
+                                % (
+                                    self.inherits.local_table.description,
+                                    self.local_table.description,
+                                )
+                            ) from afe
+                    assert self.inherits.persist_selectable is not None
+                    self.persist_selectable = sql.join(
+                        self.inherits.persist_selectable,
+                        self.local_table,
+                        self.inherit_condition,
+                    )
+
+                    fks = util.to_set(self.inherit_foreign_keys)
+                    self._inherits_equated_pairs = sql_util.criterion_as_pairs(
+                        self.persist_selectable.onclause,
+                        consider_as_foreign_keys=fks,
+                    )
+            else:
+                self.persist_selectable = self.local_table
+
+            if self.polymorphic_identity is None:
+                self._identity_class = self.class_
+
+                if (
+                    not self.polymorphic_abstract
+                    and self.inherits.base_mapper.polymorphic_on is not None
+                ):
+                    util.warn(
+                        f"{self} does not indicate a 'polymorphic_identity', "
+                        "yet is part of an inheritance hierarchy that has a "
+                        f"'polymorphic_on' column of "
+                        f"'{self.inherits.base_mapper.polymorphic_on}'. "
+                        "If this is an intermediary class that should not be "
+                        "instantiated, the class may either be left unmapped, "
+                        "or may include the 'polymorphic_abstract=True' "
+                        "parameter in its Mapper arguments. To leave the "
+                        "class unmapped when using Declarative, set the "
+                        "'__abstract__ = True' attribute on the class."
+                    )
+            elif self.concrete:
+                self._identity_class = self.class_
+            else:
+                self._identity_class = self.inherits._identity_class
+
+            if self.version_id_col is None:
+                self.version_id_col = self.inherits.version_id_col
+                self.version_id_generator = self.inherits.version_id_generator
+            elif (
+                self.inherits.version_id_col is not None
+                and self.version_id_col is not self.inherits.version_id_col
+            ):
+                util.warn(
+                    "Inheriting version_id_col '%s' does not match inherited "
+                    "version_id_col '%s' and will not automatically populate "
+                    "the inherited versioning column. "
+                    "version_id_col should only be specified on "
+                    "the base-most mapper that includes versioning."
+                    % (
+                        self.version_id_col.description,
+                        self.inherits.version_id_col.description,
+                    )
+                )
+
+            self.polymorphic_map = self.inherits.polymorphic_map
+            self.batch = self.inherits.batch
+            self.inherits._inheriting_mappers.append(self)
+            self.base_mapper = self.inherits.base_mapper
+            self.passive_updates = self.inherits.passive_updates
+            self.passive_deletes = (
+                self.inherits.passive_deletes or self.passive_deletes
+            )
+            self._all_tables = self.inherits._all_tables
+
+            if self.polymorphic_identity is not None:
+                if self.polymorphic_identity in self.polymorphic_map:
+                    util.warn(
+                        "Reassigning polymorphic association for identity %r "
+                        "from %r to %r: Check for duplicate use of %r as "
+                        "value for polymorphic_identity."
+                        % (
+                            self.polymorphic_identity,
+                            self.polymorphic_map[self.polymorphic_identity],
+                            self,
+                            self.polymorphic_identity,
+                        )
+                    )
+                self.polymorphic_map[self.polymorphic_identity] = self
+
+            if self.polymorphic_load and self.concrete:
+                raise sa_exc.ArgumentError(
+                    "polymorphic_load is not currently supported "
+                    "with concrete table inheritance"
+                )
+            if self.polymorphic_load == "inline":
+                self.inherits._add_with_polymorphic_subclass(self)
+            elif self.polymorphic_load == "selectin":
+                pass
+            elif self.polymorphic_load is not None:
+                raise sa_exc.ArgumentError(
+                    "unknown argument for polymorphic_load: %r"
+                    % self.polymorphic_load
+                )
+
+        else:
+            self._all_tables = set()
+            self.base_mapper = self
+            assert self.local_table is not None
+            self.persist_selectable = self.local_table
+            if self.polymorphic_identity is not None:
+                self.polymorphic_map[self.polymorphic_identity] = self
+            self._identity_class = self.class_
+
+        if self.persist_selectable is None:
+            raise sa_exc.ArgumentError(
+                "Mapper '%s' does not have a persist_selectable specified."
+                % self
+            )
+
+    def _set_with_polymorphic(
+        self, with_polymorphic: Optional[_WithPolymorphicArg]
+    ) -> None:
+        if with_polymorphic == "*":
+            self.with_polymorphic = ("*", None)
+        elif isinstance(with_polymorphic, (tuple, list)):
+            if isinstance(with_polymorphic[0], (str, tuple, list)):
+                self.with_polymorphic = cast(
+                    """Tuple[
+                        Union[
+                            Literal["*"],
+                            Sequence[Union["Mapper[Any]", Type[Any]]],
+                        ],
+                        Optional["FromClause"],
+                    ]""",
+                    with_polymorphic,
+                )
+            else:
+                self.with_polymorphic = (with_polymorphic, None)
+        elif with_polymorphic is not None:
+            raise sa_exc.ArgumentError(
+                f"Invalid setting for with_polymorphic: {with_polymorphic!r}"
+            )
+        else:
+            self.with_polymorphic = None
+
+        if self.with_polymorphic and self.with_polymorphic[1] is not None:
+            self.with_polymorphic = (
+                self.with_polymorphic[0],
+                coercions.expect(
+                    roles.StrictFromClauseRole,
+                    self.with_polymorphic[1],
+                    allow_select=True,
+                ),
+            )
+
+        if self.configured:
+            self._expire_memoizations()
+
+    def _add_with_polymorphic_subclass(self, mapper):
+        subcl = mapper.class_
+        if self.with_polymorphic is None:
+            self._set_with_polymorphic((subcl,))
+        elif self.with_polymorphic[0] != "*":
+            assert isinstance(self.with_polymorphic[0], tuple)
+            self._set_with_polymorphic(
+                (self.with_polymorphic[0] + (subcl,), self.with_polymorphic[1])
+            )
+
+    def _set_concrete_base(self, mapper):
+        """Set the given :class:`_orm.Mapper` as the 'inherits' for this
+        :class:`_orm.Mapper`, assuming this :class:`_orm.Mapper` is concrete
+        and does not already have an inherits."""
+
+        assert self.concrete
+        assert not self.inherits
+        assert isinstance(mapper, Mapper)
+        self.inherits = mapper
+        self.inherits.polymorphic_map.update(self.polymorphic_map)
+        self.polymorphic_map = self.inherits.polymorphic_map
+        for mapper in self.iterate_to_root():
+            if mapper.polymorphic_on is not None:
+                mapper._requires_row_aliasing = True
+        self.batch = self.inherits.batch
+        for mp in self.self_and_descendants:
+            mp.base_mapper = self.inherits.base_mapper
+        self.inherits._inheriting_mappers.append(self)
+        self.passive_updates = self.inherits.passive_updates
+        self._all_tables = self.inherits._all_tables
+
+        for key, prop in mapper._props.items():
+            if key not in self._props and not self._should_exclude(
+                key, key, local=False, column=None
+            ):
+                self._adapt_inherited_property(key, prop, False)
+
+    def _set_polymorphic_on(self, polymorphic_on):
+        self.polymorphic_on = polymorphic_on
+        self._configure_polymorphic_setter(True)
+
+    def _configure_class_instrumentation(self):
+        """If this mapper is to be a primary mapper (i.e. the
+        non_primary flag is not set), associate this Mapper with the
+        given class and entity name.
+
+        Subsequent calls to ``class_mapper()`` for the ``class_`` / ``entity``
+        name combination will return this mapper.  Also decorate the
+        `__init__` method on the mapped class to include optional
+        auto-session attachment logic.
+
+        """
+
+        # we expect that declarative has applied the class manager
+        # already and set up a registry.  if this is None,
+        # this raises as of 2.0.
+        manager = attributes.opt_manager_of_class(self.class_)
+
+        if self.non_primary:
+            if not manager or not manager.is_mapped:
+                raise sa_exc.InvalidRequestError(
+                    "Class %s has no primary mapper configured.  Configure "
+                    "a primary mapper first before setting up a non primary "
+                    "Mapper." % self.class_
+                )
+            self.class_manager = manager
+
+            assert manager.registry is not None
+            self.registry = manager.registry
+            self._identity_class = manager.mapper._identity_class
+            manager.registry._add_non_primary_mapper(self)
+            return
+
+        if manager is None or not manager.registry:
+            raise sa_exc.InvalidRequestError(
+                "The _mapper() function and Mapper() constructor may not be "
+                "invoked directly outside of a declarative registry."
+                " Please use the sqlalchemy.orm.registry.map_imperatively() "
+                "function for a classical mapping."
+            )
+
+        self.dispatch.instrument_class(self, self.class_)
+
+        # this invokes the class_instrument event and sets up
+        # the __init__ method.  documented behavior is that this must
+        # occur after the instrument_class event above.
+        # yes two events with the same two words reversed and different APIs.
+        # :(
+
+        manager = instrumentation.register_class(
+            self.class_,
+            mapper=self,
+            expired_attribute_loader=util.partial(
+                loading.load_scalar_attributes, self
+            ),
+            # finalize flag means instrument the __init__ method
+            # and call the class_instrument event
+            finalize=True,
+        )
+
+        self.class_manager = manager
+
+        assert manager.registry is not None
+        self.registry = manager.registry
+
+        # The remaining members can be added by any mapper,
+        # e_name None or not.
+        if manager.mapper is None:
+            return
+
+        event.listen(manager, "init", _event_on_init, raw=True)
+
+        for key, method in util.iterate_attributes(self.class_):
+            if key == "__init__" and hasattr(method, "_sa_original_init"):
+                method = method._sa_original_init
+                if hasattr(method, "__func__"):
+                    method = method.__func__
+            if callable(method):
+                if hasattr(method, "__sa_reconstructor__"):
+                    self._reconstructor = method
+                    event.listen(manager, "load", _event_on_load, raw=True)
+                elif hasattr(method, "__sa_validators__"):
+                    validation_opts = method.__sa_validation_opts__
+                    for name in method.__sa_validators__:
+                        if name in self.validators:
+                            raise sa_exc.InvalidRequestError(
+                                "A validation function for mapped "
+                                "attribute %r on mapper %s already exists."
+                                % (name, self)
+                            )
+                        self.validators = self.validators.union(
+                            {name: (method, validation_opts)}
+                        )
+
+    def _set_dispose_flags(self) -> None:
+        self.configured = True
+        self._ready_for_configure = True
+        self._dispose_called = True
+
+        self.__dict__.pop("_configure_failed", None)
+
+    def _str_arg_to_mapped_col(self, argname: str, key: str) -> Column[Any]:
+        try:
+            prop = self._props[key]
+        except KeyError as err:
+            raise sa_exc.ArgumentError(
+                f"Can't determine {argname} column '{key}' - "
+                "no attribute is mapped to this name."
+            ) from err
+        try:
+            expr = prop.expression
+        except AttributeError as ae:
+            raise sa_exc.ArgumentError(
+                f"Can't determine {argname} column '{key}'; "
+                "property does not refer to a single mapped Column"
+            ) from ae
+        if not isinstance(expr, Column):
+            raise sa_exc.ArgumentError(
+                f"Can't determine {argname} column '{key}'; "
+                "property does not refer to a single "
+                "mapped Column"
+            )
+        return expr
+
+    def _configure_pks(self) -> None:
+        self.tables = sql_util.find_tables(self.persist_selectable)
+
+        self._all_tables.update(t for t in self.tables)
+
+        self._pks_by_table = {}
+        self._cols_by_table = {}
+
+        all_cols = util.column_set(
+            chain(*[col.proxy_set for col in self._columntoproperty])
+        )
+
+        pk_cols = util.column_set(c for c in all_cols if c.primary_key)
+
+        # identify primary key columns which are also mapped by this mapper.
+        for fc in set(self.tables).union([self.persist_selectable]):
+            if fc.primary_key and pk_cols.issuperset(fc.primary_key):
+                # ordering is important since it determines the ordering of
+                # mapper.primary_key (and therefore query.get())
+                self._pks_by_table[fc] = util.ordered_column_set(  # type: ignore  # noqa: E501
+                    fc.primary_key
+                ).intersection(
+                    pk_cols
+                )
+            self._cols_by_table[fc] = util.ordered_column_set(fc.c).intersection(  # type: ignore  # noqa: E501
+                all_cols
+            )
+
+        if self._primary_key_argument:
+            coerced_pk_arg = [
+                (
+                    self._str_arg_to_mapped_col("primary_key", c)
+                    if isinstance(c, str)
+                    else c
+                )
+                for c in (
+                    coercions.expect(
+                        roles.DDLConstraintColumnRole,
+                        coerce_pk,
+                        argname="primary_key",
+                    )
+                    for coerce_pk in self._primary_key_argument
+                )
+            ]
+        else:
+            coerced_pk_arg = None
+
+        # if explicit PK argument sent, add those columns to the
+        # primary key mappings
+        if coerced_pk_arg:
+            for k in coerced_pk_arg:
+                if k.table not in self._pks_by_table:
+                    self._pks_by_table[k.table] = util.OrderedSet()
+                self._pks_by_table[k.table].add(k)
+
+        # otherwise, see that we got a full PK for the mapped table
+        elif (
+            self.persist_selectable not in self._pks_by_table
+            or len(self._pks_by_table[self.persist_selectable]) == 0
+        ):
+            raise sa_exc.ArgumentError(
+                "Mapper %s could not assemble any primary "
+                "key columns for mapped table '%s'"
+                % (self, self.persist_selectable.description)
+            )
+        elif self.local_table not in self._pks_by_table and isinstance(
+            self.local_table, schema.Table
+        ):
+            util.warn(
+                "Could not assemble any primary "
+                "keys for locally mapped table '%s' - "
+                "no rows will be persisted in this Table."
+                % self.local_table.description
+            )
+
+        if (
+            self.inherits
+            and not self.concrete
+            and not self._primary_key_argument
+        ):
+            # if inheriting, the "primary key" for this mapper is
+            # that of the inheriting (unless concrete or explicit)
+            self.primary_key = self.inherits.primary_key
+        else:
+            # determine primary key from argument or persist_selectable pks
+            primary_key: Collection[ColumnElement[Any]]
+
+            if coerced_pk_arg:
+                primary_key = [
+                    cc if cc is not None else c
+                    for cc, c in (
+                        (self.persist_selectable.corresponding_column(c), c)
+                        for c in coerced_pk_arg
+                    )
+                ]
+            else:
+                # if heuristically determined PKs, reduce to the minimal set
+                # of columns by eliminating FK->PK pairs for a multi-table
+                # expression.   May over-reduce for some kinds of UNIONs
+                # / CTEs; use explicit PK argument for these special cases
+                primary_key = sql_util.reduce_columns(
+                    self._pks_by_table[self.persist_selectable],
+                    ignore_nonexistent_tables=True,
+                )
+
+            if len(primary_key) == 0:
+                raise sa_exc.ArgumentError(
+                    "Mapper %s could not assemble any primary "
+                    "key columns for mapped table '%s'"
+                    % (self, self.persist_selectable.description)
+                )
+
+            self.primary_key = tuple(primary_key)
+            self._log("Identified primary key columns: %s", primary_key)
+
+        # determine cols that aren't expressed within our tables; mark these
+        # as "read only" properties which are refreshed upon INSERT/UPDATE
+        self._readonly_props = {
+            self._columntoproperty[col]
+            for col in self._columntoproperty
+            if self._columntoproperty[col] not in self._identity_key_props
+            and (
+                not hasattr(col, "table")
+                or col.table not in self._cols_by_table
+            )
+        }
+
+    def _configure_properties(self) -> None:
+        self.columns = self.c = sql_base.ColumnCollection()  # type: ignore
+
+        # object attribute names mapped to MapperProperty objects
+        self._props = util.OrderedDict()
+
+        # table columns mapped to MapperProperty
+        self._columntoproperty = _ColumnMapping(self)
+
+        explicit_col_props_by_column: Dict[
+            KeyedColumnElement[Any], Tuple[str, ColumnProperty[Any]]
+        ] = {}
+        explicit_col_props_by_key: Dict[str, ColumnProperty[Any]] = {}
+
+        # step 1: go through properties that were explicitly passed
+        # in the properties dictionary.  For Columns that are local, put them
+        # aside in a separate collection we will reconcile with the Table
+        # that's given.  For other properties, set them up in _props now.
+        if self._init_properties:
+            for key, prop_arg in self._init_properties.items():
+                if not isinstance(prop_arg, MapperProperty):
+                    possible_col_prop = self._make_prop_from_column(
+                        key, prop_arg
+                    )
+                else:
+                    possible_col_prop = prop_arg
+
+                # issue #8705.  if the explicit property is actually a
+                # Column that is local to the local Table, don't set it up
+                # in ._props yet, integrate it into the order given within
+                # the Table.
+
+                _map_as_property_now = True
+                if isinstance(possible_col_prop, properties.ColumnProperty):
+                    for given_col in possible_col_prop.columns:
+                        if self.local_table.c.contains_column(given_col):
+                            _map_as_property_now = False
+                            explicit_col_props_by_key[key] = possible_col_prop
+                            explicit_col_props_by_column[given_col] = (
+                                key,
+                                possible_col_prop,
+                            )
+
+                if _map_as_property_now:
+                    self._configure_property(
+                        key,
+                        possible_col_prop,
+                        init=False,
+                    )
+
+        # step 2: pull properties from the inherited mapper.  reconcile
+        # columns with those which are explicit above.  for properties that
+        # are only in the inheriting mapper, set them up as local props
+        if self.inherits:
+            for key, inherited_prop in self.inherits._props.items():
+                if self._should_exclude(key, key, local=False, column=None):
+                    continue
+
+                incoming_prop = explicit_col_props_by_key.get(key)
+                if incoming_prop:
+                    new_prop = self._reconcile_prop_with_incoming_columns(
+                        key,
+                        inherited_prop,
+                        warn_only=False,
+                        incoming_prop=incoming_prop,
+                    )
+                    explicit_col_props_by_key[key] = new_prop
+
+                    for inc_col in incoming_prop.columns:
+                        explicit_col_props_by_column[inc_col] = (
+                            key,
+                            new_prop,
+                        )
+                elif key not in self._props:
+                    self._adapt_inherited_property(key, inherited_prop, False)
+
+        # step 3.  Iterate through all columns in the persist selectable.
+        # this includes not only columns in the local table / fromclause,
+        # but also those columns in the superclass table if we are joined
+        # inh or single inh mapper.  map these columns as well. additional
+        # reconciliation against inherited columns occurs here also.
+
+        for column in self.persist_selectable.columns:
+            if column in explicit_col_props_by_column:
+                # column was explicitly passed to properties; configure
+                # it now in the order in which it corresponds to the
+                # Table / selectable
+                key, prop = explicit_col_props_by_column[column]
+                self._configure_property(key, prop, init=False)
+                continue
+
+            elif column in self._columntoproperty:
+                continue
+
+            column_key = (self.column_prefix or "") + column.key
+            if self._should_exclude(
+                column.key,
+                column_key,
+                local=self.local_table.c.contains_column(column),
+                column=column,
+            ):
+                continue
+
+            # adjust the "key" used for this column to that
+            # of the inheriting mapper
+            for mapper in self.iterate_to_root():
+                if column in mapper._columntoproperty:
+                    column_key = mapper._columntoproperty[column].key
+
+            self._configure_property(
+                column_key,
+                column,
+                init=False,
+                setparent=True,
+            )
+
+    def _configure_polymorphic_setter(self, init=False):
+        """Configure an attribute on the mapper representing the
+        'polymorphic_on' column, if applicable, and not
+        already generated by _configure_properties (which is typical).
+
+        Also create a setter function which will assign this
+        attribute to the value of the 'polymorphic_identity'
+        upon instance construction, also if applicable.  This
+        routine will run when an instance is created.
+
+        """
+        setter = False
+        polymorphic_key: Optional[str] = None
+
+        if self.polymorphic_on is not None:
+            setter = True
+
+            if isinstance(self.polymorphic_on, str):
+                # polymorphic_on specified as a string - link
+                # it to mapped ColumnProperty
+                try:
+                    self.polymorphic_on = self._props[self.polymorphic_on]
+                except KeyError as err:
+                    raise sa_exc.ArgumentError(
+                        "Can't determine polymorphic_on "
+                        "value '%s' - no attribute is "
+                        "mapped to this name." % self.polymorphic_on
+                    ) from err
+
+            if self.polymorphic_on in self._columntoproperty:
+                # polymorphic_on is a column that is already mapped
+                # to a ColumnProperty
+                prop = self._columntoproperty[self.polymorphic_on]
+            elif isinstance(self.polymorphic_on, MapperProperty):
+                # polymorphic_on is directly a MapperProperty,
+                # ensure it's a ColumnProperty
+                if not isinstance(
+                    self.polymorphic_on, properties.ColumnProperty
+                ):
+                    raise sa_exc.ArgumentError(
+                        "Only direct column-mapped "
+                        "property or SQL expression "
+                        "can be passed for polymorphic_on"
+                    )
+                prop = self.polymorphic_on
+            else:
+                # polymorphic_on is a Column or SQL expression and
+                # doesn't appear to be mapped. this means it can be 1.
+                # only present in the with_polymorphic selectable or
+                # 2. a totally standalone SQL expression which we'd
+                # hope is compatible with this mapper's persist_selectable
+                col = self.persist_selectable.corresponding_column(
+                    self.polymorphic_on
+                )
+                if col is None:
+                    # polymorphic_on doesn't derive from any
+                    # column/expression isn't present in the mapped
+                    # table. we will make a "hidden" ColumnProperty
+                    # for it. Just check that if it's directly a
+                    # schema.Column and we have with_polymorphic, it's
+                    # likely a user error if the schema.Column isn't
+                    # represented somehow in either persist_selectable or
+                    # with_polymorphic.   Otherwise as of 0.7.4 we
+                    # just go with it and assume the user wants it
+                    # that way (i.e. a CASE statement)
+                    setter = False
+                    instrument = False
+                    col = self.polymorphic_on
+                    if isinstance(col, schema.Column) and (
+                        self.with_polymorphic is None
+                        or self.with_polymorphic[1] is None
+                        or self.with_polymorphic[1].corresponding_column(col)
+                        is None
+                    ):
+                        raise sa_exc.InvalidRequestError(
+                            "Could not map polymorphic_on column "
+                            "'%s' to the mapped table - polymorphic "
+                            "loads will not function properly"
+                            % col.description
+                        )
+                else:
+                    # column/expression that polymorphic_on derives from
+                    # is present in our mapped table
+                    # and is probably mapped, but polymorphic_on itself
+                    # is not.  This happens when
+                    # the polymorphic_on is only directly present in the
+                    # with_polymorphic selectable, as when use
+                    # polymorphic_union.
+                    # we'll make a separate ColumnProperty for it.
+                    instrument = True
+                key = getattr(col, "key", None)
+                if key:
+                    if self._should_exclude(key, key, False, col):
+                        raise sa_exc.InvalidRequestError(
+                            "Cannot exclude or override the "
+                            "discriminator column %r" % key
+                        )
+                else:
+                    self.polymorphic_on = col = col.label("_sa_polymorphic_on")
+                    key = col.key
+
+                prop = properties.ColumnProperty(col, _instrument=instrument)
+                self._configure_property(key, prop, init=init, setparent=True)
+
+            # the actual polymorphic_on should be the first public-facing
+            # column in the property
+            self.polymorphic_on = prop.columns[0]
+            polymorphic_key = prop.key
+        else:
+            # no polymorphic_on was set.
+            # check inheriting mappers for one.
+            for mapper in self.iterate_to_root():
+                # determine if polymorphic_on of the parent
+                # should be propagated here.   If the col
+                # is present in our mapped table, or if our mapped
+                # table is the same as the parent (i.e. single table
+                # inheritance), we can use it
+                if mapper.polymorphic_on is not None:
+                    if self.persist_selectable is mapper.persist_selectable:
+                        self.polymorphic_on = mapper.polymorphic_on
+                    else:
+                        self.polymorphic_on = (
+                            self.persist_selectable
+                        ).corresponding_column(mapper.polymorphic_on)
+                    # we can use the parent mapper's _set_polymorphic_identity
+                    # directly; it ensures the polymorphic_identity of the
+                    # instance's mapper is used so is portable to subclasses.
+                    if self.polymorphic_on is not None:
+                        self._set_polymorphic_identity = (
+                            mapper._set_polymorphic_identity
+                        )
+                        self._polymorphic_attr_key = (
+                            mapper._polymorphic_attr_key
+                        )
+                        self._validate_polymorphic_identity = (
+                            mapper._validate_polymorphic_identity
+                        )
+                    else:
+                        self._set_polymorphic_identity = None
+                        self._polymorphic_attr_key = None
+                    return
+
+        if self.polymorphic_abstract and self.polymorphic_on is None:
+            raise sa_exc.InvalidRequestError(
+                "The Mapper.polymorphic_abstract parameter may only be used "
+                "on a mapper hierarchy which includes the "
+                "Mapper.polymorphic_on parameter at the base of the hierarchy."
+            )
+
+        if setter:
+
+            def _set_polymorphic_identity(state):
+                dict_ = state.dict
+                # TODO: what happens if polymorphic_on column attribute name
+                # does not match .key?
+
+                polymorphic_identity = (
+                    state.manager.mapper.polymorphic_identity
+                )
+                if (
+                    polymorphic_identity is None
+                    and state.manager.mapper.polymorphic_abstract
+                ):
+                    raise sa_exc.InvalidRequestError(
+                        f"Can't instantiate class for {state.manager.mapper}; "
+                        "mapper is marked polymorphic_abstract=True"
+                    )
+
+                state.get_impl(polymorphic_key).set(
+                    state,
+                    dict_,
+                    polymorphic_identity,
+                    None,
+                )
+
+            self._polymorphic_attr_key = polymorphic_key
+
+            def _validate_polymorphic_identity(mapper, state, dict_):
+                if (
+                    polymorphic_key in dict_
+                    and dict_[polymorphic_key]
+                    not in mapper._acceptable_polymorphic_identities
+                ):
+                    util.warn_limited(
+                        "Flushing object %s with "
+                        "incompatible polymorphic identity %r; the "
+                        "object may not refresh and/or load correctly",
+                        (state_str(state), dict_[polymorphic_key]),
+                    )
+
+            self._set_polymorphic_identity = _set_polymorphic_identity
+            self._validate_polymorphic_identity = (
+                _validate_polymorphic_identity
+            )
+        else:
+            self._polymorphic_attr_key = None
+            self._set_polymorphic_identity = None
+
+    _validate_polymorphic_identity = None
+
+    @HasMemoized.memoized_attribute
+    def _version_id_prop(self):
+        if self.version_id_col is not None:
+            return self._columntoproperty[self.version_id_col]
+        else:
+            return None
+
+    @HasMemoized.memoized_attribute
+    def _acceptable_polymorphic_identities(self):
+        identities = set()
+
+        stack = deque([self])
+        while stack:
+            item = stack.popleft()
+            if item.persist_selectable is self.persist_selectable:
+                identities.add(item.polymorphic_identity)
+                stack.extend(item._inheriting_mappers)
+
+        return identities
+
+    @HasMemoized.memoized_attribute
+    def _prop_set(self):
+        return frozenset(self._props.values())
+
+    @util.preload_module("sqlalchemy.orm.descriptor_props")
+    def _adapt_inherited_property(self, key, prop, init):
+        descriptor_props = util.preloaded.orm_descriptor_props
+
+        if not self.concrete:
+            self._configure_property(key, prop, init=False, setparent=False)
+        elif key not in self._props:
+            # determine if the class implements this attribute; if not,
+            # or if it is implemented by the attribute that is handling the
+            # given superclass-mapped property, then we need to report that we
+            # can't use this at the instance level since we are a concrete
+            # mapper and we don't map this.  don't trip user-defined
+            # descriptors that might have side effects when invoked.
+            implementing_attribute = self.class_manager._get_class_attr_mro(
+                key, prop
+            )
+            if implementing_attribute is prop or (
+                isinstance(
+                    implementing_attribute, attributes.InstrumentedAttribute
+                )
+                and implementing_attribute._parententity is prop.parent
+            ):
+                self._configure_property(
+                    key,
+                    descriptor_props.ConcreteInheritedProperty(),
+                    init=init,
+                    setparent=True,
+                )
+
+    @util.preload_module("sqlalchemy.orm.descriptor_props")
+    def _configure_property(
+        self,
+        key: str,
+        prop_arg: Union[KeyedColumnElement[Any], MapperProperty[Any]],
+        *,
+        init: bool = True,
+        setparent: bool = True,
+        warn_for_existing: bool = False,
+    ) -> MapperProperty[Any]:
+        descriptor_props = util.preloaded.orm_descriptor_props
+        self._log(
+            "_configure_property(%s, %s)", key, prop_arg.__class__.__name__
+        )
+
+        if not isinstance(prop_arg, MapperProperty):
+            prop: MapperProperty[Any] = self._property_from_column(
+                key, prop_arg
+            )
+        else:
+            prop = prop_arg
+
+        if isinstance(prop, properties.ColumnProperty):
+            col = self.persist_selectable.corresponding_column(prop.columns[0])
+
+            # if the column is not present in the mapped table,
+            # test if a column has been added after the fact to the
+            # parent table (or their parent, etc.) [ticket:1570]
+            if col is None and self.inherits:
+                path = [self]
+                for m in self.inherits.iterate_to_root():
+                    col = m.local_table.corresponding_column(prop.columns[0])
+                    if col is not None:
+                        for m2 in path:
+                            m2.persist_selectable._refresh_for_new_column(col)
+                        col = self.persist_selectable.corresponding_column(
+                            prop.columns[0]
+                        )
+                        break
+                    path.append(m)
+
+            # subquery expression, column not present in the mapped
+            # selectable.
+            if col is None:
+                col = prop.columns[0]
+
+                # column is coming in after _readonly_props was
+                # initialized; check for 'readonly'
+                if hasattr(self, "_readonly_props") and (
+                    not hasattr(col, "table")
+                    or col.table not in self._cols_by_table
+                ):
+                    self._readonly_props.add(prop)
+
+            else:
+                # if column is coming in after _cols_by_table was
+                # initialized, ensure the col is in the right set
+                if (
+                    hasattr(self, "_cols_by_table")
+                    and col.table in self._cols_by_table
+                    and col not in self._cols_by_table[col.table]
+                ):
+                    self._cols_by_table[col.table].add(col)
+
+            # if this properties.ColumnProperty represents the "polymorphic
+            # discriminator" column, mark it.  We'll need this when rendering
+            # columns in SELECT statements.
+            if not hasattr(prop, "_is_polymorphic_discriminator"):
+                prop._is_polymorphic_discriminator = (
+                    col is self.polymorphic_on
+                    or prop.columns[0] is self.polymorphic_on
+                )
+
+            if isinstance(col, expression.Label):
+                # new in 1.4, get column property against expressions
+                # to be addressable in subqueries
+                col.key = col._tq_key_label = key
+
+            self.columns.add(col, key)
+
+            for col in prop.columns:
+                for proxy_col in col.proxy_set:
+                    self._columntoproperty[proxy_col] = prop
+
+        if getattr(prop, "key", key) != key:
+            util.warn(
+                f"ORM mapped property {self.class_.__name__}.{prop.key} being "
+                "assigned to attribute "
+                f"{key!r} is already associated with "
+                f"attribute {prop.key!r}. The attribute will be de-associated "
+                f"from {prop.key!r}."
+            )
+
+        prop.key = key
+
+        if setparent:
+            prop.set_parent(self, init)
+
+        if key in self._props and getattr(
+            self._props[key], "_mapped_by_synonym", False
+        ):
+            syn = self._props[key]._mapped_by_synonym
+            raise sa_exc.ArgumentError(
+                "Can't call map_column=True for synonym %r=%r, "
+                "a ColumnProperty already exists keyed to the name "
+                "%r for column %r" % (syn, key, key, syn)
+            )
+
+        # replacement cases
+
+        # case one: prop is replacing a prop that we have mapped.  this is
+        # independent of whatever might be in the actual class dictionary
+        if (
+            key in self._props
+            and not isinstance(
+                self._props[key], descriptor_props.ConcreteInheritedProperty
+            )
+            and not isinstance(prop, descriptor_props.SynonymProperty)
+        ):
+            if warn_for_existing:
+                util.warn_deprecated(
+                    f"User-placed attribute {self.class_.__name__}.{key} on "
+                    f"{self} is replacing an existing ORM-mapped attribute.  "
+                    "Behavior is not fully defined in this case.  This "
+                    "use is deprecated and will raise an error in a future "
+                    "release",
+                    "2.0",
+                )
+            oldprop = self._props[key]
+            self._path_registry.pop(oldprop, None)
+
+        # case two: prop is replacing an attribute on the class of some kind.
+        # we have to be more careful here since it's normal when using
+        # Declarative that all the "declared attributes" on the class
+        # get replaced.
+        elif (
+            warn_for_existing
+            and self.class_.__dict__.get(key, None) is not None
+            and not isinstance(prop, descriptor_props.SynonymProperty)
+            and not isinstance(
+                self._props.get(key, None),
+                descriptor_props.ConcreteInheritedProperty,
+            )
+        ):
+            util.warn_deprecated(
+                f"User-placed attribute {self.class_.__name__}.{key} on "
+                f"{self} is replacing an existing class-bound "
+                "attribute of the same name.  "
+                "Behavior is not fully defined in this case.  This "
+                "use is deprecated and will raise an error in a future "
+                "release",
+                "2.0",
+            )
+
+        self._props[key] = prop
+
+        if not self.non_primary:
+            prop.instrument_class(self)
+
+        for mapper in self._inheriting_mappers:
+            mapper._adapt_inherited_property(key, prop, init)
+
+        if init:
+            prop.init()
+            prop.post_instrument_class(self)
+
+        if self.configured:
+            self._expire_memoizations()
+
+        return prop
+
+    def _make_prop_from_column(
+        self,
+        key: str,
+        column: Union[
+            Sequence[KeyedColumnElement[Any]], KeyedColumnElement[Any]
+        ],
+    ) -> ColumnProperty[Any]:
+        columns = util.to_list(column)
+        mapped_column = []
+        for c in columns:
+            mc = self.persist_selectable.corresponding_column(c)
+            if mc is None:
+                mc = self.local_table.corresponding_column(c)
+                if mc is not None:
+                    # if the column is in the local table but not the
+                    # mapped table, this corresponds to adding a
+                    # column after the fact to the local table.
+                    # [ticket:1523]
+                    self.persist_selectable._refresh_for_new_column(mc)
+                mc = self.persist_selectable.corresponding_column(c)
+                if mc is None:
+                    raise sa_exc.ArgumentError(
+                        "When configuring property '%s' on %s, "
+                        "column '%s' is not represented in the mapper's "
+                        "table. Use the `column_property()` function to "
+                        "force this column to be mapped as a read-only "
+                        "attribute." % (key, self, c)
+                    )
+            mapped_column.append(mc)
+        return properties.ColumnProperty(*mapped_column)
+
+    def _reconcile_prop_with_incoming_columns(
+        self,
+        key: str,
+        existing_prop: MapperProperty[Any],
+        warn_only: bool,
+        incoming_prop: Optional[ColumnProperty[Any]] = None,
+        single_column: Optional[KeyedColumnElement[Any]] = None,
+    ) -> ColumnProperty[Any]:
+        if incoming_prop and (
+            self.concrete
+            or not isinstance(existing_prop, properties.ColumnProperty)
+        ):
+            return incoming_prop
+
+        existing_column = existing_prop.columns[0]
+
+        if incoming_prop and existing_column in incoming_prop.columns:
+            return incoming_prop
+
+        if incoming_prop is None:
+            assert single_column is not None
+            incoming_column = single_column
+            equated_pair_key = (existing_prop.columns[0], incoming_column)
+        else:
+            assert single_column is None
+            incoming_column = incoming_prop.columns[0]
+            equated_pair_key = (incoming_column, existing_prop.columns[0])
+
+        if (
+            (
+                not self._inherits_equated_pairs
+                or (equated_pair_key not in self._inherits_equated_pairs)
+            )
+            and not existing_column.shares_lineage(incoming_column)
+            and existing_column is not self.version_id_col
+            and incoming_column is not self.version_id_col
+        ):
+            msg = (
+                "Implicitly combining column %s with column "
+                "%s under attribute '%s'.  Please configure one "
+                "or more attributes for these same-named columns "
+                "explicitly."
+                % (
+                    existing_prop.columns[-1],
+                    incoming_column,
+                    key,
+                )
+            )
+            if warn_only:
+                util.warn(msg)
+            else:
+                raise sa_exc.InvalidRequestError(msg)
+
+        # existing properties.ColumnProperty from an inheriting
+        # mapper. make a copy and append our column to it
+        # breakpoint()
+        new_prop = existing_prop.copy()
+
+        new_prop.columns.insert(0, incoming_column)
+        self._log(
+            "inserting column to existing list "
+            "in properties.ColumnProperty %s",
+            key,
+        )
+        return new_prop  # type: ignore
+
+    @util.preload_module("sqlalchemy.orm.descriptor_props")
+    def _property_from_column(
+        self,
+        key: str,
+        column: KeyedColumnElement[Any],
+    ) -> ColumnProperty[Any]:
+        """generate/update a :class:`.ColumnProperty` given a
+        :class:`_schema.Column` or other SQL expression object."""
+
+        descriptor_props = util.preloaded.orm_descriptor_props
+
+        prop = self._props.get(key)
+
+        if isinstance(prop, properties.ColumnProperty):
+            return self._reconcile_prop_with_incoming_columns(
+                key,
+                prop,
+                single_column=column,
+                warn_only=prop.parent is not self,
+            )
+        elif prop is None or isinstance(
+            prop, descriptor_props.ConcreteInheritedProperty
+        ):
+            return self._make_prop_from_column(key, column)
+        else:
+            raise sa_exc.ArgumentError(
+                "WARNING: when configuring property '%s' on %s, "
+                "column '%s' conflicts with property '%r'. "
+                "To resolve this, map the column to the class under a "
+                "different name in the 'properties' dictionary.  Or, "
+                "to remove all awareness of the column entirely "
+                "(including its availability as a foreign key), "
+                "use the 'include_properties' or 'exclude_properties' "
+                "mapper arguments to control specifically which table "
+                "columns get mapped." % (key, self, column.key, prop)
+            )
+
+    @util.langhelpers.tag_method_for_warnings(
+        "This warning originated from the `configure_mappers()` process, "
+        "which was invoked automatically in response to a user-initiated "
+        "operation.",
+        sa_exc.SAWarning,
+    )
+    def _check_configure(self) -> None:
+        if self.registry._new_mappers:
+            _configure_registries({self.registry}, cascade=True)
+
+    def _post_configure_properties(self) -> None:
+        """Call the ``init()`` method on all ``MapperProperties``
+        attached to this mapper.
+
+        This is a deferred configuration step which is intended
+        to execute once all mappers have been constructed.
+
+        """
+
+        self._log("_post_configure_properties() started")
+        l = [(key, prop) for key, prop in self._props.items()]
+        for key, prop in l:
+            self._log("initialize prop %s", key)
+
+            if prop.parent is self and not prop._configure_started:
+                prop.init()
+
+            if prop._configure_finished:
+                prop.post_instrument_class(self)
+
+        self._log("_post_configure_properties() complete")
+        self.configured = True
+
+    def add_properties(self, dict_of_properties):
+        """Add the given dictionary of properties to this mapper,
+        using `add_property`.
+
+        """
+        for key, value in dict_of_properties.items():
+            self.add_property(key, value)
+
+    def add_property(
+        self, key: str, prop: Union[Column[Any], MapperProperty[Any]]
+    ) -> None:
+        """Add an individual MapperProperty to this mapper.
+
+        If the mapper has not been configured yet, just adds the
+        property to the initial properties dictionary sent to the
+        constructor.  If this Mapper has already been configured, then
+        the given MapperProperty is configured immediately.
+
+        """
+        prop = self._configure_property(
+            key, prop, init=self.configured, warn_for_existing=True
+        )
+        assert isinstance(prop, MapperProperty)
+        self._init_properties[key] = prop
+
+    def _expire_memoizations(self) -> None:
+        for mapper in self.iterate_to_root():
+            mapper._reset_memoizations()
+
+    @property
+    def _log_desc(self) -> str:
+        return (
+            "("
+            + self.class_.__name__
+            + "|"
+            + (
+                self.local_table is not None
+                and self.local_table.description
+                or str(self.local_table)
+            )
+            + (self.non_primary and "|non-primary" or "")
+            + ")"
+        )
+
+    def _log(self, msg: str, *args: Any) -> None:
+        self.logger.info("%s " + msg, *((self._log_desc,) + args))
+
+    def _log_debug(self, msg: str, *args: Any) -> None:
+        self.logger.debug("%s " + msg, *((self._log_desc,) + args))
+
+    def __repr__(self) -> str:
+        return "<Mapper at 0x%x; %s>" % (id(self), self.class_.__name__)
+
+    def __str__(self) -> str:
+        return "Mapper[%s%s(%s)]" % (
+            self.class_.__name__,
+            self.non_primary and " (non-primary)" or "",
+            (
+                self.local_table.description
+                if self.local_table is not None
+                else self.persist_selectable.description
+            ),
+        )
+
+    def _is_orphan(self, state: InstanceState[_O]) -> bool:
+        orphan_possible = False
+        for mapper in self.iterate_to_root():
+            for key, cls in mapper._delete_orphans:
+                orphan_possible = True
+
+                has_parent = attributes.manager_of_class(cls).has_parent(
+                    state, key, optimistic=state.has_identity
+                )
+
+                if self.legacy_is_orphan and has_parent:
+                    return False
+                elif not self.legacy_is_orphan and not has_parent:
+                    return True
+
+        if self.legacy_is_orphan:
+            return orphan_possible
+        else:
+            return False
+
+    def has_property(self, key: str) -> bool:
+        return key in self._props
+
+    def get_property(
+        self, key: str, _configure_mappers: bool = False
+    ) -> MapperProperty[Any]:
+        """return a MapperProperty associated with the given key."""
+
+        if _configure_mappers:
+            self._check_configure()
+
+        try:
+            return self._props[key]
+        except KeyError as err:
+            raise sa_exc.InvalidRequestError(
+                f"Mapper '{self}' has no property '{key}'.  If this property "
+                "was indicated from other mappers or configure events, ensure "
+                "registry.configure() has been called."
+            ) from err
+
+    def get_property_by_column(
+        self, column: ColumnElement[_T]
+    ) -> MapperProperty[_T]:
+        """Given a :class:`_schema.Column` object, return the
+        :class:`.MapperProperty` which maps this column."""
+
+        return self._columntoproperty[column]
+
+    @property
+    def iterate_properties(self):
+        """return an iterator of all MapperProperty objects."""
+
+        return iter(self._props.values())
+
+    def _mappers_from_spec(
+        self, spec: Any, selectable: Optional[FromClause]
+    ) -> Sequence[Mapper[Any]]:
+        """given a with_polymorphic() argument, return the set of mappers it
+        represents.
+
+        Trims the list of mappers to just those represented within the given
+        selectable, if present. This helps some more legacy-ish mappings.
+
+        """
+        if spec == "*":
+            mappers = list(self.self_and_descendants)
+        elif spec:
+            mapper_set = set()
+            for m in util.to_list(spec):
+                m = _class_to_mapper(m)
+                if not m.isa(self):
+                    raise sa_exc.InvalidRequestError(
+                        "%r does not inherit from %r" % (m, self)
+                    )
+
+                if selectable is None:
+                    mapper_set.update(m.iterate_to_root())
+                else:
+                    mapper_set.add(m)
+            mappers = [m for m in self.self_and_descendants if m in mapper_set]
+        else:
+            mappers = []
+
+        if selectable is not None:
+            tables = set(
+                sql_util.find_tables(selectable, include_aliases=True)
+            )
+            mappers = [m for m in mappers if m.local_table in tables]
+        return mappers
+
+    def _selectable_from_mappers(
+        self, mappers: Iterable[Mapper[Any]], innerjoin: bool
+    ) -> FromClause:
+        """given a list of mappers (assumed to be within this mapper's
+        inheritance hierarchy), construct an outerjoin amongst those mapper's
+        mapped tables.
+
+        """
+        from_obj = self.persist_selectable
+        for m in mappers:
+            if m is self:
+                continue
+            if m.concrete:
+                raise sa_exc.InvalidRequestError(
+                    "'with_polymorphic()' requires 'selectable' argument "
+                    "when concrete-inheriting mappers are used."
+                )
+            elif not m.single:
+                if innerjoin:
+                    from_obj = from_obj.join(
+                        m.local_table, m.inherit_condition
+                    )
+                else:
+                    from_obj = from_obj.outerjoin(
+                        m.local_table, m.inherit_condition
+                    )
+
+        return from_obj
+
+    @HasMemoized.memoized_attribute
+    def _version_id_has_server_side_value(self) -> bool:
+        vid_col = self.version_id_col
+
+        if vid_col is None:
+            return False
+
+        elif not isinstance(vid_col, Column):
+            return True
+        else:
+            return vid_col.server_default is not None or (
+                vid_col.default is not None
+                and (
+                    not vid_col.default.is_scalar
+                    and not vid_col.default.is_callable
+                )
+            )
+
+    @HasMemoized.memoized_attribute
+    def _single_table_criterion(self):
+        if self.single and self.inherits and self.polymorphic_on is not None:
+            return self.polymorphic_on._annotate(
+                {"parententity": self, "parentmapper": self}
+            ).in_(
+                [
+                    m.polymorphic_identity
+                    for m in self.self_and_descendants
+                    if not m.polymorphic_abstract
+                ]
+            )
+        else:
+            return None
+
+    @HasMemoized.memoized_attribute
+    def _has_aliased_polymorphic_fromclause(self):
+        """return True if with_polymorphic[1] is an aliased fromclause,
+        like a subquery.
+
+        As of #8168, polymorphic adaption with ORMAdapter is used only
+        if this is present.
+
+        """
+        return self.with_polymorphic and isinstance(
+            self.with_polymorphic[1],
+            expression.AliasedReturnsRows,
+        )
+
+    @HasMemoized.memoized_attribute
+    def _should_select_with_poly_adapter(self):
+        """determine if _MapperEntity or _ORMColumnEntity will need to use
+        polymorphic adaption when setting up a SELECT as well as fetching
+        rows for mapped classes and subclasses against this Mapper.
+
+        moved here from context.py for #8456 to generalize the ruleset
+        for this condition.
+
+        """
+
+        # this has been simplified as of #8456.
+        # rule is: if we have a with_polymorphic or a concrete-style
+        # polymorphic selectable, *or* if the base mapper has either of those,
+        # we turn on the adaption thing.  if not, we do *no* adaption.
+        #
+        # (UPDATE for #8168: the above comment was not accurate, as we were
+        # still saying "do polymorphic" if we were using an auto-generated
+        # flattened JOIN for with_polymorphic.)
+        #
+        # this splits the behavior among the "regular" joined inheritance
+        # and single inheritance mappers, vs. the "weird / difficult"
+        # concrete and joined inh mappings that use a with_polymorphic of
+        # some kind or polymorphic_union.
+        #
+        # note we have some tests in test_polymorphic_rel that query against
+        # a subclass, then refer to the superclass that has a with_polymorphic
+        # on it (such as test_join_from_polymorphic_explicit_aliased_three).
+        # these tests actually adapt the polymorphic selectable (like, the
+        # UNION or the SELECT subquery with JOIN in it) to be just the simple
+        # subclass table.   Hence even if we are a "plain" inheriting mapper
+        # but our base has a wpoly on it, we turn on adaption.  This is a
+        # legacy case we should probably disable.
+        #
+        #
+        # UPDATE: simplified way more as of #8168.   polymorphic adaption
+        # is turned off even if with_polymorphic is set, as long as there
+        # is no user-defined aliased selectable / subquery configured.
+        # this scales back the use of polymorphic adaption in practice
+        # to basically no cases except for concrete inheritance with a
+        # polymorphic base class.
+        #
+        return (
+            self._has_aliased_polymorphic_fromclause
+            or self._requires_row_aliasing
+            or (self.base_mapper._has_aliased_polymorphic_fromclause)
+            or self.base_mapper._requires_row_aliasing
+        )
+
+    @HasMemoized.memoized_attribute
+    def _with_polymorphic_mappers(self) -> Sequence[Mapper[Any]]:
+        self._check_configure()
+
+        if not self.with_polymorphic:
+            return []
+        return self._mappers_from_spec(*self.with_polymorphic)
+
+    @HasMemoized.memoized_attribute
+    def _post_inspect(self):
+        """This hook is invoked by attribute inspection.
+
+        E.g. when Query calls:
+
+            coercions.expect(roles.ColumnsClauseRole, ent, keep_inspect=True)
+
+        This allows the inspection process run a configure mappers hook.
+
+        """
+        self._check_configure()
+
+    @HasMemoized_ro_memoized_attribute
+    def _with_polymorphic_selectable(self) -> FromClause:
+        if not self.with_polymorphic:
+            return self.persist_selectable
+
+        spec, selectable = self.with_polymorphic
+        if selectable is not None:
+            return selectable
+        else:
+            return self._selectable_from_mappers(
+                self._mappers_from_spec(spec, selectable), False
+            )
+
+    with_polymorphic_mappers = _with_polymorphic_mappers
+    """The list of :class:`_orm.Mapper` objects included in the
+    default "polymorphic" query.
+
+    """
+
+    @HasMemoized_ro_memoized_attribute
+    def _insert_cols_evaluating_none(self):
+        return {
+            table: frozenset(
+                col for col in columns if col.type.should_evaluate_none
+            )
+            for table, columns in self._cols_by_table.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _insert_cols_as_none(self):
+        return {
+            table: frozenset(
+                col.key
+                for col in columns
+                if not col.primary_key
+                and not col.server_default
+                and not col.default
+                and not col.type.should_evaluate_none
+            )
+            for table, columns in self._cols_by_table.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _propkey_to_col(self):
+        return {
+            table: {self._columntoproperty[col].key: col for col in columns}
+            for table, columns in self._cols_by_table.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _pk_keys_by_table(self):
+        return {
+            table: frozenset([col.key for col in pks])
+            for table, pks in self._pks_by_table.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _pk_attr_keys_by_table(self):
+        return {
+            table: frozenset([self._columntoproperty[col].key for col in pks])
+            for table, pks in self._pks_by_table.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _server_default_cols(
+        self,
+    ) -> Mapping[FromClause, FrozenSet[Column[Any]]]:
+        return {
+            table: frozenset(
+                [
+                    col
+                    for col in cast("Iterable[Column[Any]]", columns)
+                    if col.server_default is not None
+                    or (
+                        col.default is not None
+                        and col.default.is_clause_element
+                    )
+                ]
+            )
+            for table, columns in self._cols_by_table.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _server_onupdate_default_cols(
+        self,
+    ) -> Mapping[FromClause, FrozenSet[Column[Any]]]:
+        return {
+            table: frozenset(
+                [
+                    col
+                    for col in cast("Iterable[Column[Any]]", columns)
+                    if col.server_onupdate is not None
+                    or (
+                        col.onupdate is not None
+                        and col.onupdate.is_clause_element
+                    )
+                ]
+            )
+            for table, columns in self._cols_by_table.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _server_default_col_keys(self) -> Mapping[FromClause, FrozenSet[str]]:
+        return {
+            table: frozenset(col.key for col in cols if col.key is not None)
+            for table, cols in self._server_default_cols.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _server_onupdate_default_col_keys(
+        self,
+    ) -> Mapping[FromClause, FrozenSet[str]]:
+        return {
+            table: frozenset(col.key for col in cols if col.key is not None)
+            for table, cols in self._server_onupdate_default_cols.items()
+        }
+
+    @HasMemoized.memoized_attribute
+    def _server_default_plus_onupdate_propkeys(self) -> Set[str]:
+        result: Set[str] = set()
+
+        col_to_property = self._columntoproperty
+        for table, columns in self._server_default_cols.items():
+            result.update(
+                col_to_property[col].key
+                for col in columns.intersection(col_to_property)
+            )
+        for table, columns in self._server_onupdate_default_cols.items():
+            result.update(
+                col_to_property[col].key
+                for col in columns.intersection(col_to_property)
+            )
+        return result
+
+    @HasMemoized.memoized_instancemethod
+    def __clause_element__(self):
+        annotations: Dict[str, Any] = {
+            "entity_namespace": self,
+            "parententity": self,
+            "parentmapper": self,
+        }
+        if self.persist_selectable is not self.local_table:
+            # joined table inheritance, with polymorphic selectable,
+            # etc.
+            annotations["dml_table"] = self.local_table._annotate(
+                {
+                    "entity_namespace": self,
+                    "parententity": self,
+                    "parentmapper": self,
+                }
+            )._set_propagate_attrs(
+                {"compile_state_plugin": "orm", "plugin_subject": self}
+            )
+
+        return self.selectable._annotate(annotations)._set_propagate_attrs(
+            {"compile_state_plugin": "orm", "plugin_subject": self}
+        )
+
+    @util.memoized_property
+    def select_identity_token(self):
+        return (
+            expression.null()
+            ._annotate(
+                {
+                    "entity_namespace": self,
+                    "parententity": self,
+                    "parentmapper": self,
+                    "identity_token": True,
+                }
+            )
+            ._set_propagate_attrs(
+                {"compile_state_plugin": "orm", "plugin_subject": self}
+            )
+        )
+
+    @property
+    def selectable(self) -> FromClause:
+        """The :class:`_schema.FromClause` construct this
+        :class:`_orm.Mapper` selects from by default.
+
+        Normally, this is equivalent to :attr:`.persist_selectable`, unless
+        the ``with_polymorphic`` feature is in use, in which case the
+        full "polymorphic" selectable is returned.
+
+        """
+        return self._with_polymorphic_selectable
+
+    def _with_polymorphic_args(
+        self,
+        spec: Any = None,
+        selectable: Union[Literal[False, None], FromClause] = False,
+        innerjoin: bool = False,
+    ) -> Tuple[Sequence[Mapper[Any]], FromClause]:
+        if selectable not in (None, False):
+            selectable = coercions.expect(
+                roles.StrictFromClauseRole, selectable, allow_select=True
+            )
+
+        if self.with_polymorphic:
+            if not spec:
+                spec = self.with_polymorphic[0]
+            if selectable is False:
+                selectable = self.with_polymorphic[1]
+        elif selectable is False:
+            selectable = None
+        mappers = self._mappers_from_spec(spec, selectable)
+        if selectable is not None:
+            return mappers, selectable
+        else:
+            return mappers, self._selectable_from_mappers(mappers, innerjoin)
+
+    @HasMemoized.memoized_attribute
+    def _polymorphic_properties(self):
+        return list(
+            self._iterate_polymorphic_properties(
+                self._with_polymorphic_mappers
+            )
+        )
+
+    @property
+    def _all_column_expressions(self):
+        poly_properties = self._polymorphic_properties
+        adapter = self._polymorphic_adapter
+
+        return [
+            adapter.columns[c] if adapter else c
+            for prop in poly_properties
+            if isinstance(prop, properties.ColumnProperty)
+            and prop._renders_in_subqueries
+            for c in prop.columns
+        ]
+
+    def _columns_plus_keys(self, polymorphic_mappers=()):
+        if polymorphic_mappers:
+            poly_properties = self._iterate_polymorphic_properties(
+                polymorphic_mappers
+            )
+        else:
+            poly_properties = self._polymorphic_properties
+
+        return [
+            (prop.key, prop.columns[0])
+            for prop in poly_properties
+            if isinstance(prop, properties.ColumnProperty)
+        ]
+
+    @HasMemoized.memoized_attribute
+    def _polymorphic_adapter(self) -> Optional[orm_util.ORMAdapter]:
+        if self._has_aliased_polymorphic_fromclause:
+            return orm_util.ORMAdapter(
+                orm_util._TraceAdaptRole.MAPPER_POLYMORPHIC_ADAPTER,
+                self,
+                selectable=self.selectable,
+                equivalents=self._equivalent_columns,
+                limit_on_entity=False,
+            )
+        else:
+            return None
+
+    def _iterate_polymorphic_properties(self, mappers=None):
+        """Return an iterator of MapperProperty objects which will render into
+        a SELECT."""
+        if mappers is None:
+            mappers = self._with_polymorphic_mappers
+
+        if not mappers:
+            for c in self.iterate_properties:
+                yield c
+        else:
+            # in the polymorphic case, filter out discriminator columns
+            # from other mappers, as these are sometimes dependent on that
+            # mapper's polymorphic selectable (which we don't want rendered)
+            for c in util.unique_list(
+                chain(
+                    *[
+                        list(mapper.iterate_properties)
+                        for mapper in [self] + mappers
+                    ]
+                )
+            ):
+                if getattr(c, "_is_polymorphic_discriminator", False) and (
+                    self.polymorphic_on is None
+                    or c.columns[0] is not self.polymorphic_on
+                ):
+                    continue
+                yield c
+
+    @HasMemoized.memoized_attribute
+    def attrs(self) -> util.ReadOnlyProperties[MapperProperty[Any]]:
+        """A namespace of all :class:`.MapperProperty` objects
+        associated this mapper.
+
+        This is an object that provides each property based on
+        its key name.  For instance, the mapper for a
+        ``User`` class which has ``User.name`` attribute would
+        provide ``mapper.attrs.name``, which would be the
+        :class:`.ColumnProperty` representing the ``name``
+        column.   The namespace object can also be iterated,
+        which would yield each :class:`.MapperProperty`.
+
+        :class:`_orm.Mapper` has several pre-filtered views
+        of this attribute which limit the types of properties
+        returned, including :attr:`.synonyms`, :attr:`.column_attrs`,
+        :attr:`.relationships`, and :attr:`.composites`.
+
+        .. warning::
+
+            The :attr:`_orm.Mapper.attrs` accessor namespace is an
+            instance of :class:`.OrderedProperties`.  This is
+            a dictionary-like object which includes a small number of
+            named methods such as :meth:`.OrderedProperties.items`
+            and :meth:`.OrderedProperties.values`.  When
+            accessing attributes dynamically, favor using the dict-access
+            scheme, e.g. ``mapper.attrs[somename]`` over
+            ``getattr(mapper.attrs, somename)`` to avoid name collisions.
+
+        .. seealso::
+
+            :attr:`_orm.Mapper.all_orm_descriptors`
+
+        """
+
+        self._check_configure()
+        return util.ReadOnlyProperties(self._props)
+
+    @HasMemoized.memoized_attribute
+    def all_orm_descriptors(self) -> util.ReadOnlyProperties[InspectionAttr]:
+        """A namespace of all :class:`.InspectionAttr` attributes associated
+        with the mapped class.
+
+        These attributes are in all cases Python :term:`descriptors`
+        associated with the mapped class or its superclasses.
+
+        This namespace includes attributes that are mapped to the class
+        as well as attributes declared by extension modules.
+        It includes any Python descriptor type that inherits from
+        :class:`.InspectionAttr`.  This includes
+        :class:`.QueryableAttribute`, as well as extension types such as
+        :class:`.hybrid_property`, :class:`.hybrid_method` and
+        :class:`.AssociationProxy`.
+
+        To distinguish between mapped attributes and extension attributes,
+        the attribute :attr:`.InspectionAttr.extension_type` will refer
+        to a constant that distinguishes between different extension types.
+
+        The sorting of the attributes is based on the following rules:
+
+        1. Iterate through the class and its superclasses in order from
+           subclass to superclass (i.e. iterate through ``cls.__mro__``)
+
+        2. For each class, yield the attributes in the order in which they
+           appear in ``__dict__``, with the exception of those in step
+           3 below.  In Python 3.6 and above this ordering will be the
+           same as that of the class' construction, with the exception
+           of attributes that were added after the fact by the application
+           or the mapper.
+
+        3. If a certain attribute key is also in the superclass ``__dict__``,
+           then it's included in the iteration for that class, and not the
+           class in which it first appeared.
+
+        The above process produces an ordering that is deterministic in terms
+        of the order in which attributes were assigned to the class.
+
+        .. versionchanged:: 1.3.19 ensured deterministic ordering for
+           :meth:`_orm.Mapper.all_orm_descriptors`.
+
+        When dealing with a :class:`.QueryableAttribute`, the
+        :attr:`.QueryableAttribute.property` attribute refers to the
+        :class:`.MapperProperty` property, which is what you get when
+        referring to the collection of mapped properties via
+        :attr:`_orm.Mapper.attrs`.
+
+        .. warning::
+
+            The :attr:`_orm.Mapper.all_orm_descriptors`
+            accessor namespace is an
+            instance of :class:`.OrderedProperties`.  This is
+            a dictionary-like object which includes a small number of
+            named methods such as :meth:`.OrderedProperties.items`
+            and :meth:`.OrderedProperties.values`.  When
+            accessing attributes dynamically, favor using the dict-access
+            scheme, e.g. ``mapper.all_orm_descriptors[somename]`` over
+            ``getattr(mapper.all_orm_descriptors, somename)`` to avoid name
+            collisions.
+
+        .. seealso::
+
+            :attr:`_orm.Mapper.attrs`
+
+        """
+        return util.ReadOnlyProperties(
+            dict(self.class_manager._all_sqla_attributes())
+        )
+
+    @HasMemoized.memoized_attribute
+    @util.preload_module("sqlalchemy.orm.descriptor_props")
+    def _pk_synonyms(self) -> Dict[str, str]:
+        """return a dictionary of {syn_attribute_name: pk_attr_name} for
+        all synonyms that refer to primary key columns
+
+        """
+        descriptor_props = util.preloaded.orm_descriptor_props
+
+        pk_keys = {prop.key for prop in self._identity_key_props}
+
+        return {
+            syn.key: syn.name
+            for k, syn in self._props.items()
+            if isinstance(syn, descriptor_props.SynonymProperty)
+            and syn.name in pk_keys
+        }
+
+    @HasMemoized.memoized_attribute
+    @util.preload_module("sqlalchemy.orm.descriptor_props")
+    def synonyms(self) -> util.ReadOnlyProperties[SynonymProperty[Any]]:
+        """Return a namespace of all :class:`.Synonym`
+        properties maintained by this :class:`_orm.Mapper`.
+
+        .. seealso::
+
+            :attr:`_orm.Mapper.attrs` - namespace of all
+            :class:`.MapperProperty`
+            objects.
+
+        """
+        descriptor_props = util.preloaded.orm_descriptor_props
+
+        return self._filter_properties(descriptor_props.SynonymProperty)
+
+    @property
+    def entity_namespace(self):
+        return self.class_
+
+    @HasMemoized.memoized_attribute
+    def column_attrs(self) -> util.ReadOnlyProperties[ColumnProperty[Any]]:
+        """Return a namespace of all :class:`.ColumnProperty`
+        properties maintained by this :class:`_orm.Mapper`.
+
+        .. seealso::
+
+            :attr:`_orm.Mapper.attrs` - namespace of all
+            :class:`.MapperProperty`
+            objects.
+
+        """
+        return self._filter_properties(properties.ColumnProperty)
+
+    @HasMemoized.memoized_attribute
+    @util.preload_module("sqlalchemy.orm.relationships")
+    def relationships(
+        self,
+    ) -> util.ReadOnlyProperties[RelationshipProperty[Any]]:
+        """A namespace of all :class:`.Relationship` properties
+        maintained by this :class:`_orm.Mapper`.
+
+        .. warning::
+
+            the :attr:`_orm.Mapper.relationships` accessor namespace is an
+            instance of :class:`.OrderedProperties`.  This is
+            a dictionary-like object which includes a small number of
+            named methods such as :meth:`.OrderedProperties.items`
+            and :meth:`.OrderedProperties.values`.  When
+            accessing attributes dynamically, favor using the dict-access
+            scheme, e.g. ``mapper.relationships[somename]`` over
+            ``getattr(mapper.relationships, somename)`` to avoid name
+            collisions.
+
+        .. seealso::
+
+            :attr:`_orm.Mapper.attrs` - namespace of all
+            :class:`.MapperProperty`
+            objects.
+
+        """
+        return self._filter_properties(
+            util.preloaded.orm_relationships.RelationshipProperty
+        )
+
+    @HasMemoized.memoized_attribute
+    @util.preload_module("sqlalchemy.orm.descriptor_props")
+    def composites(self) -> util.ReadOnlyProperties[CompositeProperty[Any]]:
+        """Return a namespace of all :class:`.Composite`
+        properties maintained by this :class:`_orm.Mapper`.
+
+        .. seealso::
+
+            :attr:`_orm.Mapper.attrs` - namespace of all
+            :class:`.MapperProperty`
+            objects.
+
+        """
+        return self._filter_properties(
+            util.preloaded.orm_descriptor_props.CompositeProperty
+        )
+
+    def _filter_properties(
+        self, type_: Type[_MP]
+    ) -> util.ReadOnlyProperties[_MP]:
+        self._check_configure()
+        return util.ReadOnlyProperties(
+            util.OrderedDict(
+                (k, v) for k, v in self._props.items() if isinstance(v, type_)
+            )
+        )
+
+    @HasMemoized.memoized_attribute
+    def _get_clause(self):
+        """create a "get clause" based on the primary key.  this is used
+        by query.get() and many-to-one lazyloads to load this item
+        by primary key.
+
+        """
+        params = [
+            (
+                primary_key,
+                sql.bindparam("pk_%d" % idx, type_=primary_key.type),
+            )
+            for idx, primary_key in enumerate(self.primary_key, 1)
+        ]
+        return (
+            sql.and_(*[k == v for (k, v) in params]),
+            util.column_dict(params),
+        )
+
+    @HasMemoized.memoized_attribute
+    def _equivalent_columns(self) -> _EquivalentColumnMap:
+        """Create a map of all equivalent columns, based on
+        the determination of column pairs that are equated to
+        one another based on inherit condition.  This is designed
+        to work with the queries that util.polymorphic_union
+        comes up with, which often don't include the columns from
+        the base table directly (including the subclass table columns
+        only).
+
+        The resulting structure is a dictionary of columns mapped
+        to lists of equivalent columns, e.g.::
+
+            {tablea.col1: {tableb.col1, tablec.col1}, tablea.col2: {tabled.col2}}
+
+        """  # noqa: E501
+        result: _EquivalentColumnMap = {}
+
+        def visit_binary(binary):
+            if binary.operator == operators.eq:
+                if binary.left in result:
+                    result[binary.left].add(binary.right)
+                else:
+                    result[binary.left] = {binary.right}
+                if binary.right in result:
+                    result[binary.right].add(binary.left)
+                else:
+                    result[binary.right] = {binary.left}
+
+        for mapper in self.base_mapper.self_and_descendants:
+            if mapper.inherit_condition is not None:
+                visitors.traverse(
+                    mapper.inherit_condition, {}, {"binary": visit_binary}
+                )
+
+        return result
+
+    def _is_userland_descriptor(self, assigned_name: str, obj: Any) -> bool:
+        if isinstance(
+            obj,
+            (
+                _MappedAttribute,
+                instrumentation.ClassManager,
+                expression.ColumnElement,
+            ),
+        ):
+            return False
+        else:
+            return assigned_name not in self._dataclass_fields
+
+    @HasMemoized.memoized_attribute
+    def _dataclass_fields(self):
+        return [f.name for f in util.dataclass_fields(self.class_)]
+
+    def _should_exclude(self, name, assigned_name, local, column):
+        """determine whether a particular property should be implicitly
+        present on the class.
+
+        This occurs when properties are propagated from an inherited class, or
+        are applied from the columns present in the mapped table.
+
+        """
+
+        if column is not None and sql_base._never_select_column(column):
+            return True
+
+        # check for class-bound attributes and/or descriptors,
+        # either local or from an inherited class
+        # ignore dataclass field default values
+        if local:
+            if self.class_.__dict__.get(
+                assigned_name, None
+            ) is not None and self._is_userland_descriptor(
+                assigned_name, self.class_.__dict__[assigned_name]
+            ):
+                return True
+        else:
+            attr = self.class_manager._get_class_attr_mro(assigned_name, None)
+            if attr is not None and self._is_userland_descriptor(
+                assigned_name, attr
+            ):
+                return True
+
+        if (
+            self.include_properties is not None
+            and name not in self.include_properties
+            and (column is None or column not in self.include_properties)
+        ):
+            self._log("not including property %s" % (name))
+            return True
+
+        if self.exclude_properties is not None and (
+            name in self.exclude_properties
+            or (column is not None and column in self.exclude_properties)
+        ):
+            self._log("excluding property %s" % (name))
+            return True
+
+        return False
+
+    def common_parent(self, other: Mapper[Any]) -> bool:
+        """Return true if the given mapper shares a
+        common inherited parent as this mapper."""
+
+        return self.base_mapper is other.base_mapper
+
+    def is_sibling(self, other: Mapper[Any]) -> bool:
+        """return true if the other mapper is an inheriting sibling to this
+        one.  common parent but different branch
+
+        """
+        return (
+            self.base_mapper is other.base_mapper
+            and not self.isa(other)
+            and not other.isa(self)
+        )
+
+    def _canload(
+        self, state: InstanceState[Any], allow_subtypes: bool
+    ) -> bool:
+        s = self.primary_mapper()
+        if self.polymorphic_on is not None or allow_subtypes:
+            return _state_mapper(state).isa(s)
+        else:
+            return _state_mapper(state) is s
+
+    def isa(self, other: Mapper[Any]) -> bool:
+        """Return True if the this mapper inherits from the given mapper."""
+
+        m: Optional[Mapper[Any]] = self
+        while m and m is not other:
+            m = m.inherits
+        return bool(m)
+
+    def iterate_to_root(self) -> Iterator[Mapper[Any]]:
+        m: Optional[Mapper[Any]] = self
+        while m:
+            yield m
+            m = m.inherits
+
+    @HasMemoized.memoized_attribute
+    def self_and_descendants(self) -> Sequence[Mapper[Any]]:
+        """The collection including this mapper and all descendant mappers.
+
+        This includes not just the immediately inheriting mappers but
+        all their inheriting mappers as well.
+
+        """
+        descendants = []
+        stack = deque([self])
+        while stack:
+            item = stack.popleft()
+            descendants.append(item)
+            stack.extend(item._inheriting_mappers)
+        return util.WeakSequence(descendants)
+
+    def polymorphic_iterator(self) -> Iterator[Mapper[Any]]:
+        """Iterate through the collection including this mapper and
+        all descendant mappers.
+
+        This includes not just the immediately inheriting mappers but
+        all their inheriting mappers as well.
+
+        To iterate through an entire hierarchy, use
+        ``mapper.base_mapper.polymorphic_iterator()``.
+
+        """
+        return iter(self.self_and_descendants)
+
+    def primary_mapper(self) -> Mapper[Any]:
+        """Return the primary mapper corresponding to this mapper's class key
+        (class)."""
+
+        return self.class_manager.mapper
+
+    @property
+    def primary_base_mapper(self) -> Mapper[Any]:
+        return self.class_manager.mapper.base_mapper
+
+    def _result_has_identity_key(self, result, adapter=None):
+        pk_cols: Sequence[ColumnClause[Any]] = self.primary_key
+        if adapter:
+            pk_cols = [adapter.columns[c] for c in pk_cols]
+        rk = result.keys()
+        for col in pk_cols:
+            if col not in rk:
+                return False
+        else:
+            return True
+
+    def identity_key_from_row(
+        self,
+        row: Union[Row[Any], RowMapping],
+        identity_token: Optional[Any] = None,
+        adapter: Optional[ORMAdapter] = None,
+    ) -> _IdentityKeyType[_O]:
+        """Return an identity-map key for use in storing/retrieving an
+        item from the identity map.
+
+        :param row: A :class:`.Row` or :class:`.RowMapping` produced from a
+         result set that selected from the ORM mapped primary key columns.
+
+         .. versionchanged:: 2.0
+            :class:`.Row` or :class:`.RowMapping` are accepted
+            for the "row" argument
+
+        """
+        pk_cols: Sequence[ColumnClause[Any]] = self.primary_key
+        if adapter:
+            pk_cols = [adapter.columns[c] for c in pk_cols]
+
+        mapping: RowMapping
+        if hasattr(row, "_mapping"):
+            mapping = row._mapping
+        else:
+            mapping = row  # type: ignore[assignment]
+
+        return (
+            self._identity_class,
+            tuple(mapping[column] for column in pk_cols),
+            identity_token,
+        )
+
+    def identity_key_from_primary_key(
+        self,
+        primary_key: Tuple[Any, ...],
+        identity_token: Optional[Any] = None,
+    ) -> _IdentityKeyType[_O]:
+        """Return an identity-map key for use in storing/retrieving an
+        item from an identity map.
+
+        :param primary_key: A list of values indicating the identifier.
+
+        """
+        return (
+            self._identity_class,
+            tuple(primary_key),
+            identity_token,
+        )
+
+    def identity_key_from_instance(self, instance: _O) -> _IdentityKeyType[_O]:
+        """Return the identity key for the given instance, based on
+        its primary key attributes.
+
+        If the instance's state is expired, calling this method
+        will result in a database check to see if the object has been deleted.
+        If the row no longer exists,
+        :class:`~sqlalchemy.orm.exc.ObjectDeletedError` is raised.
+
+        This value is typically also found on the instance state under the
+        attribute name `key`.
+
+        """
+        state = attributes.instance_state(instance)
+        return self._identity_key_from_state(state, PassiveFlag.PASSIVE_OFF)
+
+    def _identity_key_from_state(
+        self,
+        state: InstanceState[_O],
+        passive: PassiveFlag = PassiveFlag.PASSIVE_RETURN_NO_VALUE,
+    ) -> _IdentityKeyType[_O]:
+        dict_ = state.dict
+        manager = state.manager
+        return (
+            self._identity_class,
+            tuple(
+                [
+                    manager[prop.key].impl.get(state, dict_, passive)
+                    for prop in self._identity_key_props
+                ]
+            ),
+            state.identity_token,
+        )
+
+    def primary_key_from_instance(self, instance: _O) -> Tuple[Any, ...]:
+        """Return the list of primary key values for the given
+        instance.
+
+        If the instance's state is expired, calling this method
+        will result in a database check to see if the object has been deleted.
+        If the row no longer exists,
+        :class:`~sqlalchemy.orm.exc.ObjectDeletedError` is raised.
+
+        """
+        state = attributes.instance_state(instance)
+        identity_key = self._identity_key_from_state(
+            state, PassiveFlag.PASSIVE_OFF
+        )
+        return identity_key[1]
+
+    @HasMemoized.memoized_attribute
+    def _persistent_sortkey_fn(self):
+        key_fns = [col.type.sort_key_function for col in self.primary_key]
+
+        if set(key_fns).difference([None]):
+
+            def key(state):
+                return tuple(
+                    key_fn(val) if key_fn is not None else val
+                    for key_fn, val in zip(key_fns, state.key[1])
+                )
+
+        else:
+
+            def key(state):
+                return state.key[1]
+
+        return key
+
+    @HasMemoized.memoized_attribute
+    def _identity_key_props(self):
+        return [self._columntoproperty[col] for col in self.primary_key]
+
+    @HasMemoized.memoized_attribute
+    def _all_pk_cols(self):
+        collection: Set[ColumnClause[Any]] = set()
+        for table in self.tables:
+            collection.update(self._pks_by_table[table])
+        return collection
+
+    @HasMemoized.memoized_attribute
+    def _should_undefer_in_wildcard(self):
+        cols: Set[ColumnElement[Any]] = set(self.primary_key)
+        if self.polymorphic_on is not None:
+            cols.add(self.polymorphic_on)
+        return cols
+
+    @HasMemoized.memoized_attribute
+    def _primary_key_propkeys(self):
+        return {self._columntoproperty[col].key for col in self._all_pk_cols}
+
+    def _get_state_attr_by_column(
+        self,
+        state: InstanceState[_O],
+        dict_: _InstanceDict,
+        column: ColumnElement[Any],
+        passive: PassiveFlag = PassiveFlag.PASSIVE_RETURN_NO_VALUE,
+    ) -> Any:
+        prop = self._columntoproperty[column]
+        return state.manager[prop.key].impl.get(state, dict_, passive=passive)
+
+    def _set_committed_state_attr_by_column(self, state, dict_, column, value):
+        prop = self._columntoproperty[column]
+        state.manager[prop.key].impl.set_committed_value(state, dict_, value)
+
+    def _set_state_attr_by_column(self, state, dict_, column, value):
+        prop = self._columntoproperty[column]
+        state.manager[prop.key].impl.set(state, dict_, value, None)
+
+    def _get_committed_attr_by_column(self, obj, column):
+        state = attributes.instance_state(obj)
+        dict_ = attributes.instance_dict(obj)
+        return self._get_committed_state_attr_by_column(
+            state, dict_, column, passive=PassiveFlag.PASSIVE_OFF
+        )
+
+    def _get_committed_state_attr_by_column(
+        self, state, dict_, column, passive=PassiveFlag.PASSIVE_RETURN_NO_VALUE
+    ):
+        prop = self._columntoproperty[column]
+        return state.manager[prop.key].impl.get_committed_value(
+            state, dict_, passive=passive
+        )
+
+    def _optimized_get_statement(self, state, attribute_names):
+        """assemble a WHERE clause which retrieves a given state by primary
+        key, using a minimized set of tables.
+
+        Applies to a joined-table inheritance mapper where the
+        requested attribute names are only present on joined tables,
+        not the base table.  The WHERE clause attempts to include
+        only those tables to minimize joins.
+
+        """
+        props = self._props
+
+        col_attribute_names = set(attribute_names).intersection(
+            state.mapper.column_attrs.keys()
+        )
+        tables: Set[FromClause] = set(
+            chain(
+                *[
+                    sql_util.find_tables(c, check_columns=True)
+                    for key in col_attribute_names
+                    for c in props[key].columns
+                ]
+            )
+        )
+
+        if self.base_mapper.local_table in tables:
+            return None
+
+        def visit_binary(binary):
+            leftcol = binary.left
+            rightcol = binary.right
+            if leftcol is None or rightcol is None:
+                return
+
+            if leftcol.table not in tables:
+                leftval = self._get_committed_state_attr_by_column(
+                    state,
+                    state.dict,
+                    leftcol,
+                    passive=PassiveFlag.PASSIVE_NO_INITIALIZE,
+                )
+                if leftval in orm_util._none_set:
+                    raise _OptGetColumnsNotAvailable()
+                binary.left = sql.bindparam(
+                    None, leftval, type_=binary.right.type
+                )
+            elif rightcol.table not in tables:
+                rightval = self._get_committed_state_attr_by_column(
+                    state,
+                    state.dict,
+                    rightcol,
+                    passive=PassiveFlag.PASSIVE_NO_INITIALIZE,
+                )
+                if rightval in orm_util._none_set:
+                    raise _OptGetColumnsNotAvailable()
+                binary.right = sql.bindparam(
+                    None, rightval, type_=binary.right.type
+                )
+
+        allconds: List[ColumnElement[bool]] = []
+
+        start = False
+
+        # as of #7507, from the lowest base table on upwards,
+        # we include all intermediary tables.
+
+        for mapper in reversed(list(self.iterate_to_root())):
+            if mapper.local_table in tables:
+                start = True
+            elif not isinstance(mapper.local_table, expression.TableClause):
+                return None
+            if start and not mapper.single:
+                assert mapper.inherits
+                assert not mapper.concrete
+                assert mapper.inherit_condition is not None
+                allconds.append(mapper.inherit_condition)
+                tables.add(mapper.local_table)
+
+        # only the bottom table needs its criteria to be altered to fit
+        # the primary key ident - the rest of the tables upwards to the
+        # descendant-most class should all be present and joined to each
+        # other.
+        try:
+            _traversed = visitors.cloned_traverse(
+                allconds[0], {}, {"binary": visit_binary}
+            )
+        except _OptGetColumnsNotAvailable:
+            return None
+        else:
+            allconds[0] = _traversed
+
+        cond = sql.and_(*allconds)
+
+        cols = []
+        for key in col_attribute_names:
+            cols.extend(props[key].columns)
+        return (
+            sql.select(*cols)
+            .where(cond)
+            .set_label_style(LABEL_STYLE_TABLENAME_PLUS_COL)
+        )
+
+    def _iterate_to_target_viawpoly(self, mapper):
+        if self.isa(mapper):
+            prev = self
+            for m in self.iterate_to_root():
+                yield m
+
+                if m is not prev and prev not in m._with_polymorphic_mappers:
+                    break
+
+                prev = m
+                if m is mapper:
+                    break
+
+    @HasMemoized.memoized_attribute
+    def _would_selectinload_combinations_cache(self):
+        return {}
+
+    def _would_selectin_load_only_from_given_mapper(self, super_mapper):
+        """return True if this mapper would "selectin" polymorphic load based
+        on the given super mapper, and not from a setting from a subclass.
+
+        given::
+
+            class A: ...
+
+
+            class B(A):
+                __mapper_args__ = {"polymorphic_load": "selectin"}
+
+
+            class C(B): ...
+
+
+            class D(B):
+                __mapper_args__ = {"polymorphic_load": "selectin"}
+
+        ``inspect(C)._would_selectin_load_only_from_given_mapper(inspect(B))``
+        returns True, because C does selectin loading because of B's setting.
+
+        OTOH, ``inspect(D)
+        ._would_selectin_load_only_from_given_mapper(inspect(B))``
+        returns False, because D does selectin loading because of its own
+        setting; when we are doing a selectin poly load from B, we want to
+        filter out D because it would already have its own selectin poly load
+        set up separately.
+
+        Added as part of #9373.
+
+        """
+        cache = self._would_selectinload_combinations_cache
+
+        try:
+            return cache[super_mapper]
+        except KeyError:
+            pass
+
+        # assert that given object is a supermapper, meaning we already
+        # strong reference it directly or indirectly.  this allows us
+        # to not worry that we are creating new strongrefs to unrelated
+        # mappers or other objects.
+        assert self.isa(super_mapper)
+
+        mapper = super_mapper
+        for m in self._iterate_to_target_viawpoly(mapper):
+            if m.polymorphic_load == "selectin":
+                retval = m is super_mapper
+                break
+        else:
+            retval = False
+
+        cache[super_mapper] = retval
+        return retval
+
+    def _should_selectin_load(self, enabled_via_opt, polymorphic_from):
+        if not enabled_via_opt:
+            # common case, takes place for all polymorphic loads
+            mapper = polymorphic_from
+            for m in self._iterate_to_target_viawpoly(mapper):
+                if m.polymorphic_load == "selectin":
+                    return m
+        else:
+            # uncommon case, selectin load options were used
+            enabled_via_opt = set(enabled_via_opt)
+            enabled_via_opt_mappers = {e.mapper: e for e in enabled_via_opt}
+            for entity in enabled_via_opt.union([polymorphic_from]):
+                mapper = entity.mapper
+                for m in self._iterate_to_target_viawpoly(mapper):
+                    if (
+                        m.polymorphic_load == "selectin"
+                        or m in enabled_via_opt_mappers
+                    ):
+                        return enabled_via_opt_mappers.get(m, m)
+
+        return None
+
+    @util.preload_module("sqlalchemy.orm.strategy_options")
+    def _subclass_load_via_in(self, entity, polymorphic_from):
+        """Assemble a that can load the columns local to
+        this subclass as a SELECT with IN.
+
+        """
+
+        strategy_options = util.preloaded.orm_strategy_options
+
+        assert self.inherits
+
+        if self.polymorphic_on is not None:
+            polymorphic_prop = self._columntoproperty[self.polymorphic_on]
+            keep_props = set([polymorphic_prop] + self._identity_key_props)
+        else:
+            keep_props = set(self._identity_key_props)
+
+        disable_opt = strategy_options.Load(entity)
+        enable_opt = strategy_options.Load(entity)
+
+        classes_to_include = {self}
+        m: Optional[Mapper[Any]] = self.inherits
+        while (
+            m is not None
+            and m is not polymorphic_from
+            and m.polymorphic_load == "selectin"
+        ):
+            classes_to_include.add(m)
+            m = m.inherits
+
+        for prop in self.column_attrs + self.relationships:
+            # skip prop keys that are not instrumented on the mapped class.
+            # this is primarily the "_sa_polymorphic_on" property that gets
+            # created for an ad-hoc polymorphic_on SQL expression, issue #8704
+            if prop.key not in self.class_manager:
+                continue
+
+            if prop.parent in classes_to_include or prop in keep_props:
+                # "enable" options, to turn on the properties that we want to
+                # load by default (subject to options from the query)
+                if not isinstance(prop, StrategizedProperty):
+                    continue
+
+                enable_opt = enable_opt._set_generic_strategy(
+                    # convert string name to an attribute before passing
+                    # to loader strategy.   note this must be in terms
+                    # of given entity, such as AliasedClass, etc.
+                    (getattr(entity.entity_namespace, prop.key),),
+                    dict(prop.strategy_key),
+                    _reconcile_to_other=True,
+                )
+            else:
+                # "disable" options, to turn off the properties from the
+                # superclass that we *don't* want to load, applied after
+                # the options from the query to override them
+                disable_opt = disable_opt._set_generic_strategy(
+                    # convert string name to an attribute before passing
+                    # to loader strategy.   note this must be in terms
+                    # of given entity, such as AliasedClass, etc.
+                    (getattr(entity.entity_namespace, prop.key),),
+                    {"do_nothing": True},
+                    _reconcile_to_other=False,
+                )
+
+        primary_key = [
+            sql_util._deep_annotate(pk, {"_orm_adapt": True})
+            for pk in self.primary_key
+        ]
+
+        in_expr: ColumnElement[Any]
+
+        if len(primary_key) > 1:
+            in_expr = sql.tuple_(*primary_key)
+        else:
+            in_expr = primary_key[0]
+
+        if entity.is_aliased_class:
+            assert entity.mapper is self
+
+            q = sql.select(entity).set_label_style(
+                LABEL_STYLE_TABLENAME_PLUS_COL
+            )
+
+            in_expr = entity._adapter.traverse(in_expr)
+            primary_key = [entity._adapter.traverse(k) for k in primary_key]
+            q = q.where(
+                in_expr.in_(sql.bindparam("primary_keys", expanding=True))
+            ).order_by(*primary_key)
+        else:
+            q = sql.select(self).set_label_style(
+                LABEL_STYLE_TABLENAME_PLUS_COL
+            )
+            q = q.where(
+                in_expr.in_(sql.bindparam("primary_keys", expanding=True))
+            ).order_by(*primary_key)
+
+        return q, enable_opt, disable_opt
+
+    @HasMemoized.memoized_attribute
+    def _subclass_load_via_in_mapper(self):
+        # the default is loading this mapper against the basemost mapper
+        return self._subclass_load_via_in(self, self.base_mapper)
+
+    def cascade_iterator(
+        self,
+        type_: str,
+        state: InstanceState[_O],
+        halt_on: Optional[Callable[[InstanceState[Any]], bool]] = None,
+    ) -> Iterator[
+        Tuple[object, Mapper[Any], InstanceState[Any], _InstanceDict]
+    ]:
+        r"""Iterate each element and its mapper in an object graph,
+        for all relationships that meet the given cascade rule.
+
+        :param type\_:
+          The name of the cascade rule (i.e. ``"save-update"``, ``"delete"``,
+          etc.).
+
+          .. note::  the ``"all"`` cascade is not accepted here.  For a generic
+             object traversal function, see :ref:`faq_walk_objects`.
+
+        :param state:
+          The lead InstanceState.  child items will be processed per
+          the relationships defined for this object's mapper.
+
+        :return: the method yields individual object instances.
+
+        .. seealso::
+
+            :ref:`unitofwork_cascades`
+
+            :ref:`faq_walk_objects` - illustrates a generic function to
+            traverse all objects without relying on cascades.
+
+        """
+        visited_states: Set[InstanceState[Any]] = set()
+        prp, mpp = object(), object()
+
+        assert state.mapper.isa(self)
+
+        # this is actually a recursive structure, fully typing it seems
+        # a little too difficult for what it's worth here
+        visitables: Deque[
+            Tuple[
+                Deque[Any],
+                object,
+                Optional[InstanceState[Any]],
+                Optional[_InstanceDict],
+            ]
+        ]
+
+        visitables = deque(
+            [(deque(state.mapper._props.values()), prp, state, state.dict)]
+        )
+
+        while visitables:
+            iterator, item_type, parent_state, parent_dict = visitables[-1]
+            if not iterator:
+                visitables.pop()
+                continue
+
+            if item_type is prp:
+                prop = iterator.popleft()
+                if not prop.cascade or type_ not in prop.cascade:
+                    continue
+                assert parent_state is not None
+                assert parent_dict is not None
+                queue = deque(
+                    prop.cascade_iterator(
+                        type_,
+                        parent_state,
+                        parent_dict,
+                        visited_states,
+                        halt_on,
+                    )
+                )
+                if queue:
+                    visitables.append((queue, mpp, None, None))
+            elif item_type is mpp:
+                (
+                    instance,
+                    instance_mapper,
+                    corresponding_state,
+                    corresponding_dict,
+                ) = iterator.popleft()
+                yield (
+                    instance,
+                    instance_mapper,
+                    corresponding_state,
+                    corresponding_dict,
+                )
+                visitables.append(
+                    (
+                        deque(instance_mapper._props.values()),
+                        prp,
+                        corresponding_state,
+                        corresponding_dict,
+                    )
+                )
+
+    @HasMemoized.memoized_attribute
+    def _compiled_cache(self):
+        return util.LRUCache(self._compiled_cache_size)
+
+    @HasMemoized.memoized_attribute
+    def _multiple_persistence_tables(self):
+        return len(self.tables) > 1
+
+    @HasMemoized.memoized_attribute
+    def _sorted_tables(self):
+        table_to_mapper: Dict[TableClause, Mapper[Any]] = {}
+
+        for mapper in self.base_mapper.self_and_descendants:
+            for t in mapper.tables:
+                table_to_mapper.setdefault(t, mapper)
+
+        extra_dependencies = []
+        for table, mapper in table_to_mapper.items():
+            super_ = mapper.inherits
+            if super_:
+                extra_dependencies.extend(
+                    [(super_table, table) for super_table in super_.tables]
+                )
+
+        def skip(fk):
+            # attempt to skip dependencies that are not
+            # significant to the inheritance chain
+            # for two tables that are related by inheritance.
+            # while that dependency may be important, it's technically
+            # not what we mean to sort on here.
+            parent = table_to_mapper.get(fk.parent.table)
+            dep = table_to_mapper.get(fk.column.table)
+            if (
+                parent is not None
+                and dep is not None
+                and dep is not parent
+                and dep.inherit_condition is not None
+            ):
+                cols = set(sql_util._find_columns(dep.inherit_condition))
+                if parent.inherit_condition is not None:
+                    cols = cols.union(
+                        sql_util._find_columns(parent.inherit_condition)
+                    )
+                    return fk.parent not in cols and fk.column not in cols
+                else:
+                    return fk.parent not in cols
+            return False
+
+        sorted_ = sql_util.sort_tables(
+            table_to_mapper,
+            skip_fn=skip,
+            extra_dependencies=extra_dependencies,
+        )
+
+        ret = util.OrderedDict()
+        for t in sorted_:
+            ret[t] = table_to_mapper[t]
+        return ret
+
+    def _memo(self, key: Any, callable_: Callable[[], _T]) -> _T:
+        if key in self._memoized_values:
+            return cast(_T, self._memoized_values[key])
+        else:
+            self._memoized_values[key] = value = callable_()
+            return value
+
+    @util.memoized_property
+    def _table_to_equated(self):
+        """memoized map of tables to collections of columns to be
+        synchronized upwards to the base mapper."""
+
+        result: util.defaultdict[
+            Table,
+            List[
+                Tuple[
+                    Mapper[Any],
+                    List[Tuple[ColumnElement[Any], ColumnElement[Any]]],
+                ]
+            ],
+        ] = util.defaultdict(list)
+
+        def set_union(x, y):
+            return x.union(y)
+
+        for table in self._sorted_tables:
+            cols = set(table.c)
+
+            for m in self.iterate_to_root():
+                if m._inherits_equated_pairs and cols.intersection(
+                    reduce(
+                        set_union,
+                        [l.proxy_set for l, r in m._inherits_equated_pairs],
+                    )
+                ):
+                    result[table].append((m, m._inherits_equated_pairs))
+
+        return result
+
+
+class _OptGetColumnsNotAvailable(Exception):
+    pass
+
+
+def configure_mappers() -> None:
+    """Initialize the inter-mapper relationships of all mappers that
+    have been constructed thus far across all :class:`_orm.registry`
+    collections.
+
+    The configure step is used to reconcile and initialize the
+    :func:`_orm.relationship` linkages between mapped classes, as well as to
+    invoke configuration events such as the
+    :meth:`_orm.MapperEvents.before_configured` and
+    :meth:`_orm.MapperEvents.after_configured`, which may be used by ORM
+    extensions or user-defined extension hooks.
+
+    Mapper configuration is normally invoked automatically, the first time
+    mappings from a particular :class:`_orm.registry` are used, as well as
+    whenever mappings are used and additional not-yet-configured mappers have
+    been constructed. The automatic configuration process however is local only
+    to the :class:`_orm.registry` involving the target mapper and any related
+    :class:`_orm.registry` objects which it may depend on; this is
+    equivalent to invoking the :meth:`_orm.registry.configure` method
+    on a particular :class:`_orm.registry`.
+
+    By contrast, the :func:`_orm.configure_mappers` function will invoke the
+    configuration process on all :class:`_orm.registry` objects that
+    exist in memory, and may be useful for scenarios where many individual
+    :class:`_orm.registry` objects that are nonetheless interrelated are
+    in use.
+
+    .. versionchanged:: 1.4
+
+        As of SQLAlchemy 1.4.0b2, this function works on a
+        per-:class:`_orm.registry` basis, locating all :class:`_orm.registry`
+        objects present and invoking the :meth:`_orm.registry.configure` method
+        on each. The :meth:`_orm.registry.configure` method may be preferred to
+        limit the configuration of mappers to those local to a particular
+        :class:`_orm.registry` and/or declarative base class.
+
+    Points at which automatic configuration is invoked include when a mapped
+    class is instantiated into an instance, as well as when ORM queries
+    are emitted using :meth:`.Session.query` or :meth:`_orm.Session.execute`
+    with an ORM-enabled statement.
+
+    The mapper configure process, whether invoked by
+    :func:`_orm.configure_mappers` or from :meth:`_orm.registry.configure`,
+    provides several event hooks that can be used to augment the mapper
+    configuration step. These hooks include:
+
+    * :meth:`.MapperEvents.before_configured` - called once before
+      :func:`.configure_mappers` or :meth:`_orm.registry.configure` does any
+      work; this can be used to establish additional options, properties, or
+      related mappings before the operation proceeds.
+
+    * :meth:`.MapperEvents.mapper_configured` - called as each individual
+      :class:`_orm.Mapper` is configured within the process; will include all
+      mapper state except for backrefs set up by other mappers that are still
+      to be configured.
+
+    * :meth:`.MapperEvents.after_configured` - called once after
+      :func:`.configure_mappers` or :meth:`_orm.registry.configure` is
+      complete; at this stage, all :class:`_orm.Mapper` objects that fall
+      within the scope of the configuration operation will be fully configured.
+      Note that the calling application may still have other mappings that
+      haven't been produced yet, such as if they are in modules as yet
+      unimported, and may also have mappings that are still to be configured,
+      if they are in other :class:`_orm.registry` collections not part of the
+      current scope of configuration.
+
+    """
+
+    _configure_registries(_all_registries(), cascade=True)
+
+
+def _configure_registries(
+    registries: Set[_RegistryType], cascade: bool
+) -> None:
+    for reg in registries:
+        if reg._new_mappers:
+            break
+    else:
+        return
+
+    with _CONFIGURE_MUTEX:
+        global _already_compiling
+        if _already_compiling:
+            return
+        _already_compiling = True
+        try:
+            # double-check inside mutex
+            for reg in registries:
+                if reg._new_mappers:
+                    break
+            else:
+                return
+
+            Mapper.dispatch._for_class(Mapper).before_configured()  # type: ignore # noqa: E501
+            # initialize properties on all mappers
+            # note that _mapper_registry is unordered, which
+            # may randomly conceal/reveal issues related to
+            # the order of mapper compilation
+
+            _do_configure_registries(registries, cascade)
+        finally:
+            _already_compiling = False
+    Mapper.dispatch._for_class(Mapper).after_configured()  # type: ignore
+
+
+@util.preload_module("sqlalchemy.orm.decl_api")
+def _do_configure_registries(
+    registries: Set[_RegistryType], cascade: bool
+) -> None:
+    registry = util.preloaded.orm_decl_api.registry
+
+    orig = set(registries)
+
+    for reg in registry._recurse_with_dependencies(registries):
+        has_skip = False
+
+        for mapper in reg._mappers_to_configure():
+            run_configure = None
+
+            for fn in mapper.dispatch.before_mapper_configured:
+                run_configure = fn(mapper, mapper.class_)
+                if run_configure is EXT_SKIP:
+                    has_skip = True
+                    break
+            if run_configure is EXT_SKIP:
+                continue
+
+            if getattr(mapper, "_configure_failed", False):
+                e = sa_exc.InvalidRequestError(
+                    "One or more mappers failed to initialize - "
+                    "can't proceed with initialization of other "
+                    "mappers. Triggering mapper: '%s'. "
+                    "Original exception was: %s"
+                    % (mapper, mapper._configure_failed)
+                )
+                e._configure_failed = mapper._configure_failed  # type: ignore
+                raise e
+
+            if not mapper.configured:
+                try:
+                    mapper._post_configure_properties()
+                    mapper._expire_memoizations()
+                    mapper.dispatch.mapper_configured(mapper, mapper.class_)
+                except Exception:
+                    exc = sys.exc_info()[1]
+                    if not hasattr(exc, "_configure_failed"):
+                        mapper._configure_failed = exc
+                    raise
+        if not has_skip:
+            reg._new_mappers = False
+
+        if not cascade and reg._dependencies.difference(orig):
+            raise sa_exc.InvalidRequestError(
+                "configure was called with cascade=False but "
+                "additional registries remain"
+            )
+
+
+@util.preload_module("sqlalchemy.orm.decl_api")
+def _dispose_registries(registries: Set[_RegistryType], cascade: bool) -> None:
+    registry = util.preloaded.orm_decl_api.registry
+
+    orig = set(registries)
+
+    for reg in registry._recurse_with_dependents(registries):
+        if not cascade and reg._dependents.difference(orig):
+            raise sa_exc.InvalidRequestError(
+                "Registry has dependent registries that are not disposed; "
+                "pass cascade=True to clear these also"
+            )
+
+        while reg._managers:
+            try:
+                manager, _ = reg._managers.popitem()
+            except KeyError:
+                # guard against race between while and popitem
+                pass
+            else:
+                reg._dispose_manager_and_mapper(manager)
+
+        reg._non_primary_mappers.clear()
+        reg._dependents.clear()
+        for dep in reg._dependencies:
+            dep._dependents.discard(reg)
+        reg._dependencies.clear()
+        # this wasn't done in the 1.3 clear_mappers() and in fact it
+        # was a bug, as it could cause configure_mappers() to invoke
+        # the "before_configured" event even though mappers had all been
+        # disposed.
+        reg._new_mappers = False
+
+
+def reconstructor(fn):
+    """Decorate a method as the 'reconstructor' hook.
+
+    Designates a single method as the "reconstructor", an ``__init__``-like
+    method that will be called by the ORM after the instance has been
+    loaded from the database or otherwise reconstituted.
+
+    .. tip::
+
+        The :func:`_orm.reconstructor` decorator makes use of the
+        :meth:`_orm.InstanceEvents.load` event hook, which can be
+        used directly.
+
+    The reconstructor will be invoked with no arguments.  Scalar
+    (non-collection) database-mapped attributes of the instance will
+    be available for use within the function.  Eagerly-loaded
+    collections are generally not yet available and will usually only
+    contain the first element.  ORM state changes made to objects at
+    this stage will not be recorded for the next flush() operation, so
+    the activity within a reconstructor should be conservative.
+
+    .. seealso::
+
+        :meth:`.InstanceEvents.load`
+
+    """
+    fn.__sa_reconstructor__ = True
+    return fn
+
+
+def validates(
+    *names: str, include_removes: bool = False, include_backrefs: bool = True
+) -> Callable[[_Fn], _Fn]:
+    r"""Decorate a method as a 'validator' for one or more named properties.
+
+    Designates a method as a validator, a method which receives the
+    name of the attribute as well as a value to be assigned, or in the
+    case of a collection, the value to be added to the collection.
+    The function can then raise validation exceptions to halt the
+    process from continuing (where Python's built-in ``ValueError``
+    and ``AssertionError`` exceptions are reasonable choices), or can
+    modify or replace the value before proceeding. The function should
+    otherwise return the given value.
+
+    Note that a validator for a collection **cannot** issue a load of that
+    collection within the validation routine - this usage raises
+    an assertion to avoid recursion overflows.  This is a reentrant
+    condition which is not supported.
+
+    :param \*names: list of attribute names to be validated.
+    :param include_removes: if True, "remove" events will be
+     sent as well - the validation function must accept an additional
+     argument "is_remove" which will be a boolean.
+
+    :param include_backrefs: defaults to ``True``; if ``False``, the
+     validation function will not emit if the originator is an attribute
+     event related via a backref.  This can be used for bi-directional
+     :func:`.validates` usage where only one validator should emit per
+     attribute operation.
+
+     .. versionchanged:: 2.0.16 This paramter inadvertently defaulted to
+        ``False`` for releases 2.0.0 through 2.0.15.  Its correct default
+        of ``True`` is restored in 2.0.16.
+
+    .. seealso::
+
+      :ref:`simple_validators` - usage examples for :func:`.validates`
+
+    """
+
+    def wrap(fn: _Fn) -> _Fn:
+        fn.__sa_validators__ = names  # type: ignore[attr-defined]
+        fn.__sa_validation_opts__ = {  # type: ignore[attr-defined]
+            "include_removes": include_removes,
+            "include_backrefs": include_backrefs,
+        }
+        return fn
+
+    return wrap
+
+
+def _event_on_load(state, ctx):
+    instrumenting_mapper = state.manager.mapper
+
+    if instrumenting_mapper._reconstructor:
+        instrumenting_mapper._reconstructor(state.obj())
+
+
+def _event_on_init(state, args, kwargs):
+    """Run init_instance hooks.
+
+    This also includes mapper compilation, normally not needed
+    here but helps with some piecemeal configuration
+    scenarios (such as in the ORM tutorial).
+
+    """
+
+    instrumenting_mapper = state.manager.mapper
+    if instrumenting_mapper:
+        instrumenting_mapper._check_configure()
+        if instrumenting_mapper._set_polymorphic_identity:
+            instrumenting_mapper._set_polymorphic_identity(state)
+
+
+class _ColumnMapping(Dict["ColumnElement[Any]", "MapperProperty[Any]"]):
+    """Error reporting helper for mapper._columntoproperty."""
+
+    __slots__ = ("mapper",)
+
+    def __init__(self, mapper):
+        # TODO: weakref would be a good idea here
+        self.mapper = mapper
+
+    def __missing__(self, column):
+        prop = self.mapper._props.get(column)
+        if prop:
+            raise orm_exc.UnmappedColumnError(
+                "Column '%s.%s' is not available, due to "
+                "conflicting property '%s':%r"
+                % (column.table.name, column.name, column.key, prop)
+            )
+        raise orm_exc.UnmappedColumnError(
+            "No column %s is configured on mapper %s..."
+            % (column, self.mapper)
+        )