Support creating SQLAlchemy sessions from a callable

Allows setting the database session through the
sqlalchemy_session_factory attribute, receiving a Callable that returns
a sqlalchemy.orm.Session instance. This ensures better control to
dynamically set the Session in the factory.

docs/changelog.rst

@@ -10,6 +10,8 @@ ChangeLog
 
     - :issue:`366`: Add :class:`factory.django.Password` to generate Django :class:`~django.contrib.auth.models.User`
       passwords.
+    - :issue:`304`: Add :attr:`~factory.alchemy.SQLAlchemyOptions.sqlalchemy_session_factory` to dynamically
+      create sessions for use by the :class:`~factory.alchemy.SQLAlchemyModelFactory`.
     - Add support for Django 3.2
     - Add support for Django 4.0
     - Add support for Python 3.10

docs/orms.rst

@@ -369,6 +369,25 @@ To work, this class needs an `SQLAlchemy`_ session object affected to the :attr:
         SQLAlchemy session to use to communicate with the database when creating
         an object through this :class:`SQLAlchemyModelFactory`.
 
+    .. attribute:: sqlalchemy_session_factory
+
+       .. versionadded:: 3.3.0
+
+         :class:`~collections.abc.Callable` returning a :class:`~sqlalchemy.orm.Session` instance to use to communicate
+         with the database. You can either provide the session through this attribute, or through
+         :attr:`~factory.alchemy.SQLAlchemyOptions.sqlalchemy_session`, but not both at the same time.
+
+        .. code-block:: python
+
+            from . import common
+
+            class UserFactory(factory.alchemy.SQLAlchemyModelFactory):
+                class Meta:
+                    model = User
+                    sqlalchemy_session_factory = lambda: common.Session()
+
+                username = 'john'
+
     .. attribute:: sqlalchemy_session_persistence
 
         Control the action taken by ``sqlalchemy_session`` at the end of a create call.

factory/alchemy.py

@@ -22,10 +22,18 @@ def _check_sqlalchemy_session_persistence(self, meta, value):
                 (meta, VALID_SESSION_PERSISTENCE_TYPES, value)
             )
 
+    @staticmethod
+    def _check_has_sqlalchemy_session_set(meta, value):
+        if value and meta.sqlalchemy_session:
+            raise RuntimeError("Provide either a sqlalchemy_session or a sqlalchemy_session_factory, not both")
+
     def _build_default_options(self):
         return super()._build_default_options() + [
             base.OptionDefault('sqlalchemy_get_or_create', (), inherit=True),
             base.OptionDefault('sqlalchemy_session', None, inherit=True),
+            base.OptionDefault(
+                'sqlalchemy_session_factory', None, inherit=True, checker=self._check_has_sqlalchemy_session_set
+            ),
             base.OptionDefault(
                 'sqlalchemy_session_persistence',
                 None,
@@ -90,6 +98,10 @@ def _get_or_create(cls, model_class, session, args, kwargs):
     @classmethod
     def _create(cls, model_class, *args, **kwargs):
         """Create an instance of the model, and save it to the database."""
+        session_factory = cls._meta.sqlalchemy_session_factory
+        if session_factory:
+            cls._meta.sqlalchemy_session = session_factory()
+
         session = cls._meta.sqlalchemy_session
 
         if session is None:

tests/test_alchemy.py

@@ -264,6 +264,34 @@ def test_build_does_not_raises_exception_when_no_session_was_set(self):
         self.assertEqual(inst1.id, 1)
 
 
+class SQLAlchemySessionFactoryTestCase(unittest.TestCase):
+
+    def test_create_get_session_from_sqlalchemy_session_factory(self):
+        class SessionGetterFactory(SQLAlchemyModelFactory):
+            class Meta:
+                model = models.StandardModel
+                sqlalchemy_session = None
+                sqlalchemy_session_factory = lambda: models.session
+
+            id = factory.Sequence(lambda n: n)
+
+        SessionGetterFactory.create()
+        self.assertEqual(SessionGetterFactory._meta.sqlalchemy_session, models.session)
+        # Reuse the session obtained from sqlalchemy_session_factory.
+        SessionGetterFactory.create()
+
+    def test_create_raise_exception_sqlalchemy_session_factory_not_callable(self):
+        message = "^Provide either a sqlalchemy_session or a sqlalchemy_session_factory, not both$"
+        with self.assertRaisesRegex(RuntimeError, message):
+            class SessionAndGetterFactory(SQLAlchemyModelFactory):
+                class Meta:
+                    model = models.StandardModel
+                    sqlalchemy_session = models.session
+                    sqlalchemy_session_factory = lambda: models.session
+
+                id = factory.Sequence(lambda n: n)
+
+
 class NameConflictTests(unittest.TestCase):
     """Regression test for `TypeError: _save() got multiple values for argument 'session'`