Merge "Raise for unexpected polymorphic identity"

2 files changed, 37 insertions, 11 deletions

diff --git a/lib/sqlalchemy/orm/loading.py b/lib/sqlalchemy/orm/loading.py
index b823de4ab..218449cdd 100644
--- a/lib/sqlalchemy/orm/loading.py
+++ b/lib/sqlalchemy/orm/loading.py
@@ -643,11 +643,9 @@ def _instance_processor(
                 identity_token,
             )
             if not is_not_primary_key(identitykey[1]):
-                raise sa_exc.InvalidRequestError(
-                    "Row with identity key %s can't be loaded into an "
-                    "object; the polymorphic discriminator column '%s' is "
-                    "NULL" % (identitykey, polymorphic_discriminator)
-                )
+                return identitykey
+            else:
+                return None
 
         _instance = _decorate_polymorphic_switch(
             _instance,
@@ -843,6 +841,8 @@ def _decorate_polymorphic_switch(
         else:
             if sub_mapper is mapper:
                 return None
+            elif not sub_mapper.isa(mapper):
+                return False
 
             return _instance_processor(
                 sub_mapper,
@@ -863,11 +863,37 @@ def _decorate_polymorphic_switch(
             _instance = polymorphic_instances[discriminator]
             if _instance:
                 return _instance(row)
+            elif _instance is False:
+                identitykey = ensure_no_pk(row)
+
+                if identitykey:
+                    raise sa_exc.InvalidRequestError(
+                        "Row with identity key %s can't be loaded into an "
+                        "object; the polymorphic discriminator column '%s' "
+                        "refers to %s, which is not a sub-mapper of "
+                        "the requested %s"
+                        % (
+                            identitykey,
+                            polymorphic_on,
+                            mapper.polymorphic_map[discriminator],
+                            mapper,
+                        )
+                    )
+                else:
+                    return None
             else:
                 return instance_fn(row)
         else:
-            ensure_no_pk(row)
-            return None
+            identitykey = ensure_no_pk(row)
+
+            if identitykey:
+                raise sa_exc.InvalidRequestError(
+                    "Row with identity key %s can't be loaded into an "
+                    "object; the polymorphic discriminator column '%s' is "
+                    "NULL" % (identitykey, polymorphic_on)
+                )
+            else:
+                return None
 
     return polymorphic_instance
 
diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index 55b569bcd..15319e049 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -260,6 +260,7 @@ class Query(Generative):
             self._from_obj_alias = sql_util.ColumnAdapter(
                 self._from_obj[0], equivs
             )
+            self._enable_single_crit = False
         elif (
             set_base_alias
             and len(self._from_obj) == 1
@@ -267,6 +268,7 @@ class Query(Generative):
             and info.is_aliased_class
         ):
             self._from_obj_alias = info._adapter
+            self._enable_single_crit = False
 
     def _reset_polymorphic_adapter(self, mapper):
         for m2 in mapper._with_polymorphic_mappers:
@@ -1379,7 +1381,6 @@ class Query(Generative):
             ._anonymous_fromclause()
         )
         q = self._from_selectable(fromclause)
-        q._enable_single_crit = False
         q._select_from_entity = self._entity_zero()
         if entities:
             q._set_entities(entities)
@@ -1407,6 +1408,7 @@ class Query(Generative):
         ):
             self.__dict__.pop(attr, None)
         self._set_select_from([fromclause], True)
+        self._enable_single_crit = False
 
         # this enables clause adaptation for non-ORM
         # expressions.
@@ -1875,9 +1877,7 @@ class Query(Generative):
             self._having = criterion
 
     def _set_op(self, expr_fn, *q):
-        return self._from_selectable(
-            expr_fn(*([self] + list(q))).subquery()
-        )._set_enable_single_crit(False)
+        return self._from_selectable(expr_fn(*([self] + list(q))).subquery())
 
     def union(self, *q):
         """Produce a UNION of this Query against one or more queries.