Ensure entity or None returned from _entity_from_pre_ent_zero()

Fixed regression where the :meth:`_orm.Query.exists` method would fail to
create an expression if the entity list of the :class:`_orm.Query` were
an arbitrary SQL column expression.

Fixes: #6076
Change-Id: I292dd5f527b2cbc1b76ca765b4ea321ef8535709

1 files changed, 7 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index 4e2b4cdeb..8ad4092e6 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -22,6 +22,7 @@ import itertools
 import operator
 import types
 
+from sqlalchemy.sql import visitors
 from . import exc as orm_exc
 from . import interfaces
 from . import loading
@@ -197,7 +198,12 @@ class Query(
         elif "bundle" in ent._annotations:
             return ent._annotations["bundle"]
         else:
-            return ent
+            # label, other SQL expression
+            for element in visitors.iterate(ent):
+                if "parententity" in element._annotations:
+                    return element._annotations["parententity"]
+            else:
+                return None
 
     def _only_full_mapper_zero(self, methname):
         if (