From e50add6ca1605dcc06c8c5a5770342779a4d5124 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] Fixed #28856 -- Fixed a regression in caching of a
 GenericForeignKey pointing to a MTI model.

Regression in b9f8635f58ad743995cad2081b3dc395e55761e5.
---
 django/db/models/fields/related.py       |  3 +++
 docs/releases/1.11.8.txt                 |  3 +++
 tests/generic_relations_regress/tests.py |  6 ++++++
 tests/model_fields/test_foreignkey.py    | 10 ++++++++++
 tests/serializers/test_xml.py            |  6 ------
 tests/serializers/tests.py               |  6 +-----
 6 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index 34123fd4de..e2e2bb23a6 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -872,6 +872,9 @@ class ForeignKey(ForeignObject):
             kwargs['to_field'] = self.remote_field.field_name
         return name, path, args, kwargs
 
+    def to_python(self, value):
+        return self.target_field.to_python(value)
+
     @property
     def target_field(self):
         return self.foreign_related_fields[0]
diff --git a/docs/releases/1.11.8.txt b/docs/releases/1.11.8.txt
index 959731bbd5..596ea434ec 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 9add025a46..cfac484053 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
diff --git a/tests/model_fields/test_foreignkey.py b/tests/model_fields/test_foreignkey.py
index 1617f7f310..7c99d8e34a 100644
--- a/tests/model_fields/test_foreignkey.py
+++ b/tests/model_fields/test_foreignkey.py
@@ -93,3 +93,13 @@ class ForeignKeyTests(TestCase):
 
         assert_app_model_resolved('model_fields')
         assert_app_model_resolved('tests')
+
+    @isolate_apps('model_fields')
+    def test_to_python(self):
+        class Foo(models.Model):
+            pass
+
+        class Bar(models.Model):
+            fk = models.ForeignKey(Foo, models.CASCADE)
+
+        self.assertEqual(Bar._meta.get_field('fk').to_python('1'), 1)
diff --git a/tests/serializers/test_xml.py b/tests/serializers/test_xml.py
index ea9677d87f..b11cfdd864 100644
--- a/tests/serializers/test_xml.py
+++ b/tests/serializers/test_xml.py
@@ -29,12 +29,6 @@ class XmlSerializerTestCase(SerializersTestBase, TestCase):
   </object>
 </django-objects>"""  # NOQA
 
-    @staticmethod
-    def _comparison_value(value):
-        # The XML serializer handles everything as strings, so comparisons
-        # need to be performed on the stringified value
-        return str(value)
-
     @staticmethod
     def _validate_output(serial_str):
         try:
diff --git a/tests/serializers/tests.py b/tests/serializers/tests.py
index 0ea60f0f64..66fc2ef40c 100644
--- a/tests/serializers/tests.py
+++ b/tests/serializers/tests.py
@@ -90,10 +90,6 @@ class SerializerRegistrationTests(SimpleTestCase):
 class SerializersTestBase:
     serializer_name = None  # Set by subclasses to the serialization format name
 
-    @staticmethod
-    def _comparison_value(value):
-        return value
-
     def setUp(self):
         sports = Category.objects.create(name="Sports")
         music = Category.objects.create(name="Music")
@@ -193,7 +189,7 @@ class SerializersTestBase:
         self.assertFalse(self._get_field_values(serial_str, 'author'))
 
         for obj in serializers.deserialize(self.serializer_name, serial_str):
-            self.assertEqual(obj.object.pk, self._comparison_value(self.joe.pk))
+            self.assertEqual(obj.object.pk, self.joe.pk)
 
     def test_serialize_field_subset(self):
         """Output can be restricted to a subset of fields"""