auth: add function to retrieve a user's group

* gn3/auth/authorisation/groups.py: new `user_group` function
* tests/unit/auth/test_groups.py: test `user_group` function

2 files changed, 38 insertions, 1 deletions

diff --git a/gn3/auth/authorisation/groups.py b/gn3/auth/authorisation/groups.py
index 6496e87..743e812 100644
--- a/gn3/auth/authorisation/groups.py
+++ b/gn3/auth/authorisation/groups.py
@@ -106,3 +106,21 @@ def authenticated_user_group(conn) -> Maybe:
         return Just(groups[0])
 
     return Nothing
+
+def user_group(conn: db.DbConnection, user: User) -> Maybe:
+    """Returns the given user's group"""
+    with db.cursor(conn) as cursor:
+        cursor.execute(
+            ("SELECT groups.group_id, groups.group_name FROM group_users "
+             "INNER JOIN groups ON group_users.group_id=groups.group_id "
+             "WHERE group_users.user_id = ?"),
+            (str(user.user_id),))
+        groups = tuple(Group(UUID(row[0]), row[1]) for row in cursor.fetchall())
+
+    if len(groups) > 1:
+        raise MembershipError(user, groups)
+
+    if len(groups) == 1:
+        return Just(groups[0])
+
+    return Nothing
diff --git a/tests/unit/auth/test_groups.py b/tests/unit/auth/test_groups.py
index 22d90f4..79dc16f 100644
--- a/tests/unit/auth/test_groups.py
+++ b/tests/unit/auth/test_groups.py
@@ -2,13 +2,14 @@
 from uuid import UUID
 
 import pytest
+from pymonad.maybe import Nothing
 
 from gn3.auth import db
 from gn3.auth.authentication.users import User
 from gn3.auth.authorisation.roles import Role
 from gn3.auth.authorisation.privileges import Privilege
 from gn3.auth.authorisation.groups import (
-    Group, GroupRole, create_group, MembershipError, create_group_role)
+    Group, GroupRole, user_group, create_group, MembershipError, create_group_role)
 
 from tests.unit.auth import conftest
 
@@ -98,3 +99,21 @@ def test_create_multiple_groups(mocker, test_app, test_users):
         # subsequent attempts should fail
         with pytest.raises(MembershipError):
             create_group(conn, "another_test_group", user)
+
+@pytest.mark.unit_test
+@pytest.mark.parametrize(
+    "user,expected",
+    tuple(zip(
+        conftest.TEST_USERS,
+        (([Group(UUID("9988c21d-f02f-4d45-8966-22c968ac2fbf"), "TheTestGroup")] * 3)
+         + [Nothing]))))
+def test_user_group(test_users_in_group, user, expected):
+    """
+    GIVEN: A bunch of registered users, some of whom are members of a group, and
+      others are not
+    WHEN: a particular user's group is requested,
+    THEN: return a Maybe containing the group that the user belongs to, or
+      Nothing
+    """
+    conn, group, users = test_users_in_group
+    assert user_group(conn, user).maybe(Nothing, lambda val: val) == expected