[2.0.x] Fixed #28856 -- Fixed a regression in caching of a GenericForeignKey pointing to a MTI model.

Regression in b9f8635f58.

Modified backport of e50add6ca1 from master

django/contrib/contenttypes/fields.py

@@ -228,12 +228,17 @@ class GenericForeignKey(FieldCacheMixin):
 
         rel_obj = self.get_cached_value(instance, default=None)
         if rel_obj is not None:
-            ct_match = ct_id == self.get_content_type(obj=rel_obj, using=instance._state.db).id
-            pk_match = rel_obj._meta.pk.to_python(pk_val) == rel_obj.pk
-            if ct_match and pk_match:
-                return rel_obj
-            else:
+            if ct_id != self.get_content_type(obj=rel_obj, using=instance._state.db).id:
                 rel_obj = None
+            else:
+                pk = rel_obj._meta.pk
+                # If the primary key is a remote field, use the referenced
+                # field's to_python().
+                pk_to_python = pk.target_field.to_python if pk.remote_field else pk.to_python
+                if pk_to_python(pk_val) != rel_obj._get_pk_val():
+                    rel_obj = None
+                else:
+                    return rel_obj
         if ct_id is not None:
             ct = self.get_content_type(id=ct_id, using=instance._state.db)
             try:

docs/releases/1.11.8.txt

@@ -27,3 +27,6 @@ Bugfixes
 
 * Made query lookups for ``CICharField``, ``CIEmailField``, and ``CITextField``
   use a ``citext`` cast (:ticket:`28702`).
+
+* Fixed a regression in caching of a ``GenericForeignKey`` when the referenced
+  model instance uses multi-table inheritance (:ticket:`28856`).

tests/generic_relations_regress/tests.py

@@ -48,6 +48,12 @@ class GenericRelationTests(TestCase):
         TextLink.objects.create(content_object=oddrel)
         oddrel.delete()
 
+    def test_coerce_object_id_remote_field_cache_persistence(self):
+        restaurant = Restaurant.objects.create()
+        CharLink.objects.create(content_object=restaurant)
+        charlink = CharLink.objects.latest('pk')
+        self.assertIs(charlink.content_object, charlink.content_object)
+
     def test_q_object_or(self):
         """
         SQL query parameters for generic relations are properly