read from cls.__dict__ so init_subclass works

Modified the :class:`.DeclarativeMeta` metaclass to pass ``cls.__dict__``
into the declarative scanning process to look for attributes, rather than
the separate dictionary passed to the type's ``__init__()`` method. This
allows user-defined base classes that add attributes within an
``__init_subclass__()`` to work as expected, as ``__init_subclass__()`` can
only affect the ``cls.__dict__`` itself and not the other dictionary. This
is technically a regression from 1.3 where ``__dict__`` was being used.

Additionally makes the reference between ClassManager and
the declarative configuration object a weak reference, so that it
can be discarded after mappers are set up.

Fixes: #7900
Change-Id: I3c2fd4e227cc1891aa4bb3d7d5b43d5686f9f27c

1 files changed, 2 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/instrumentation.py b/lib/sqlalchemy/orm/instrumentation.py
index a5dc305d2..0d4b630da 100644
--- a/lib/sqlalchemy/orm/instrumentation.py
+++ b/lib/sqlalchemy/orm/instrumentation.py
@@ -39,6 +39,7 @@ from typing import Optional
 from typing import Set
 from typing import TYPE_CHECKING
 from typing import TypeVar
+import weakref
 
 from . import base
 from . import collections
@@ -167,7 +168,7 @@ class ClassManager(
         if registry:
             registry._add_manager(self)
         if declarative_scan:
-            self.declarative_scan = declarative_scan
+            self.declarative_scan = weakref.ref(declarative_scan)
         if expired_attribute_loader:
             self.expired_attribute_loader = expired_attribute_loader