From d31424fec1a3de9d281535c0503644a9d7b93c63 Mon Sep 17 00:00:00 2001
From: Simon Charette <charette.s@gmail.com>
Date: Wed, 29 Nov 2017 01:06:45 -0500
Subject: [PATCH] [2.0.x] Fixed #28856 -- Fixed a regression in caching of a
 GenericForeignKey pointing to a MTI model.

Regression in b9f8635f58ad743995cad2081b3dc395e55761e5.

Modified backport of e50add6ca1605dcc06c8c5a5770342779a4d5124 from master
---
 django/contrib/contenttypes/fields.py    | 15 ++++++++++-----
 docs/releases/1.11.8.txt                 |  3 +++
 tests/generic_relations_regress/tests.py |  6 ++++++
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/django/contrib/contenttypes/fields.py b/django/contrib/contenttypes/fields.py
index 22277072031..42e06aa119e 100644
--- a/django/contrib/contenttypes/fields.py
+++ b/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:
diff --git a/docs/releases/1.11.8.txt b/docs/releases/1.11.8.txt
index 959731bbd5c..596ea434ece 100644
--- a/docs/releases/1.11.8.txt
+++ b/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`).
diff --git a/tests/generic_relations_regress/tests.py b/tests/generic_relations_regress/tests.py
index 9add025a46a..cfac484053a 100644
--- a/tests/generic_relations_regress/tests.py
+++ b/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