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

6 files changed, 19 insertions, 3 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index a959b0a40..5ca2858e9 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -34,6 +34,7 @@ from .base import NO_CHANGE  # noqa
 from .base import NO_RAISE
 from .base import NO_VALUE
 from .base import NON_PERSISTENT_OK  # noqa
+from .base import PASSIVE_CLASS_MISMATCH  # noqa
 from .base import PASSIVE_NO_FETCH
 from .base import PASSIVE_NO_FETCH_RELATED  # noqa
 from .base import PASSIVE_NO_INITIALIZE
diff --git a/lib/sqlalchemy/orm/base.py b/lib/sqlalchemy/orm/base.py
index a31745aec..6df8acf1e 100644
--- a/lib/sqlalchemy/orm/base.py
+++ b/lib/sqlalchemy/orm/base.py
@@ -25,6 +25,13 @@ PASSIVE_NO_RESULT = util.symbol(
     """,
 )
 
+PASSIVE_CLASS_MISMATCH = util.symbol(
+    "PASSIVE_CLASS_MISMATCH",
+    """Symbol indicating that an object is locally present for a given
+    primary key identity but it is not of the requested class.  The
+    return value is therefore None and no SQL should be emitted.""",
+)
+
 ATTR_WAS_SET = util.symbol(
     "ATTR_WAS_SET",
     """Symbol returned by a loader callable to indicate the
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:
diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index fe1ea9bfc..0da7d08a4 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -1048,6 +1048,8 @@ class Query(Generative):
                 if not issubclass(instance.__class__, mapper.class_):
                     return None
                 return instance
+            elif instance is attributes.PASSIVE_CLASS_MISMATCH:
+                return None
 
         return db_load_fn(self, primary_key_identity)
 
diff --git a/lib/sqlalchemy/orm/session.py b/lib/sqlalchemy/orm/session.py
index f172649ba..fefdd4ef1 100644
--- a/lib/sqlalchemy/orm/session.py
+++ b/lib/sqlalchemy/orm/session.py
@@ -1617,7 +1617,7 @@ class Session(_SessionClassMethods):
         key = mapper.identity_key_from_primary_key(
             primary_key_identity, identity_token=identity_token
         )
-        return loading.get_from_identity(self, key, passive)
+        return loading.get_from_identity(self, mapper, key, passive)
 
     @property
     @util.contextmanager
diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 7d05a37f5..094637082 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -774,7 +774,10 @@ class LazyLoader(AbstractRelationshipLoader, util.MemoizedSlots):
             )
 
             if instance is not None:
-                return instance
+                if instance is attributes.PASSIVE_CLASS_MISMATCH:
+                    return None
+                else:
+                    return instance
             elif (
                 not passive & attributes.SQL_OK
                 or not passive & attributes.RELATED_OBJECT_OK