Test instance for matching class hierarchy on get_from_identity

Fixed issue where a lazyload that uses session-local "get" against a target
many-to-one relationship where an object with the correct primary key is
present, however it's an instance of a sibling class, does not correctly
return None as is the case when the lazy loader actually emits a load for
that row.

Fixes: #5210
Change-Id: I89f9946cfeba61d89a272435f76a5a082b1da30c

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/loading.py b/lib/sqlalchemy/orm/loading.py
index c3d4773cd..49c71e5b2 100644
--- a/lib/sqlalchemy/orm/loading.py
+++ b/lib/sqlalchemy/orm/loading.py
@@ -161,7 +161,7 @@ def merge_result(query, iterator, load=True):
         session.autoflush = autoflush
 
 
-def get_from_identity(session, key, passive):
+def get_from_identity(session, mapper, key, passive):
     """Look up the given key in the given session's identity map,
     check the object for expired state if found.
 
@@ -171,6 +171,9 @@ def get_from_identity(session, key, passive):
 
         state = attributes.instance_state(instance)
 
+        if mapper.inherits and not state.mapper.isa(mapper):
+            return attributes.PASSIVE_CLASS_MISMATCH
+
         # expired - ensure it still exists
         if state.expired:
             if not passive & attributes.SQL_OK: