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

Regression in b9f8635f58.

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]

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

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)

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:

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"""