Raise informative error when non-object m2o comparison used

An informative error message is raised when an ORM many-to-one comparison
is attempted against an object that is not an actual mapped instance.
Comparisons such as those to scalar subqueries aren't supported;
generalized comparison with subqueries is better achieved using
:meth:`~.RelationshipProperty.Comparator.has`.

Fixes: #5269
Change-Id: I2e23178eb59728c39241a46bfa7411239a87492e

1 files changed, 12 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/relationships.py b/lib/sqlalchemy/orm/relationships.py
index 7d33c4649..6ac56a324 100644
--- a/lib/sqlalchemy/orm/relationships.py
+++ b/lib/sqlalchemy/orm/relationships.py
@@ -1621,8 +1621,19 @@ class RelationshipProperty(StrategizedProperty):
         alias_secondary=True,
     ):
         if state is not None:
-            state = attributes.instance_state(state)
+            try:
+                state = inspect(state)
+            except sa_exc.NoInspectionAvailable:
+                state = None
 
+            if state is None or not getattr(state, "is_instance", False):
+                raise sa_exc.ArgumentError(
+                    "Mapped instance expected for relationship "
+                    "comparison to object.   Classes, queries and other "
+                    "SQL elements are not accepted in this context; for "
+                    "comparison with a subquery, "
+                    "use %s.has(**criteria)." % self
+                )
         reverse_direction = not value_is_parent
 
         if state is None: