From b9f8635f58ad743995cad2081b3dc395e55761e5 Mon Sep 17 00:00:00 2001
From: Michal Petrucha <michal.petrucha@koniiiik.org>
Date: Sun, 1 May 2016 18:32:40 +0200
Subject: [PATCH] Refs #16508 -- Added invalidation of stale cached instances
 of GenericForeignKey targets.

---
 django/contrib/contenttypes/fields.py | 39 ++++++++++++++++-----------
 tests/generic_relations/tests.py      | 17 ++++++++++++
 2 files changed, 41 insertions(+), 15 deletions(-)

diff --git a/django/contrib/contenttypes/fields.py b/django/contrib/contenttypes/fields.py
index e4bc292642..99633a5c67 100644
--- a/django/contrib/contenttypes/fields.py
+++ b/django/contrib/contenttypes/fields.py
@@ -208,7 +208,7 @@ class GenericForeignKey(object):
                 lambda obj: (obj._get_pk_val(), obj.__class__),
                 gfk_key,
                 True,
-                self.cache_attr)
+                self.name)
 
     def is_cached(self, instance):
         return hasattr(instance, self.cache_attr)
@@ -217,26 +217,35 @@ class GenericForeignKey(object):
         if instance is None:
             return self
 
+        # Don't use getattr(instance, self.ct_field) here because that might
+        # reload the same ContentType over and over (#5570). Instead, get the
+        # content type ID here, and later when the actual instance is needed,
+        # use ContentType.objects.get_for_id(), which has a global cache.
+        f = self.model._meta.get_field(self.ct_field)
+        ct_id = getattr(instance, f.get_attname(), None)
+        pk_val = getattr(instance, self.fk_field)
+
         try:
-            return getattr(instance, self.cache_attr)
+            rel_obj = getattr(instance, self.cache_attr)
         except AttributeError:
             rel_obj = None
+        else:
+            if rel_obj and (ct_id != self.get_content_type(obj=rel_obj, using=instance._state.db).id or
+                            rel_obj._meta.pk.to_python(pk_val) != rel_obj._get_pk_val()):
+                rel_obj = None
 
-            # Make sure to use ContentType.objects.get_for_id() to ensure that
-            # lookups are cached (see ticket #5570). This takes more code than
-            # the naive ``getattr(instance, self.ct_field)``, but has better
-            # performance when dealing with GFKs in loops and such.
-            f = self.model._meta.get_field(self.ct_field)
-            ct_id = getattr(instance, f.get_attname(), None)
-            if ct_id is not None:
-                ct = self.get_content_type(id=ct_id, using=instance._state.db)
-                try:
-                    rel_obj = ct.get_object_for_this_type(pk=getattr(instance, self.fk_field))
-                except ObjectDoesNotExist:
-                    pass
-            setattr(instance, self.cache_attr, rel_obj)
+        if rel_obj is not None:
             return rel_obj
 
+        if ct_id is not None:
+            ct = self.get_content_type(id=ct_id, using=instance._state.db)
+            try:
+                rel_obj = ct.get_object_for_this_type(pk=pk_val)
+            except ObjectDoesNotExist:
+                pass
+        setattr(instance, self.cache_attr, rel_obj)
+        return rel_obj
+
     def __set__(self, instance, value):
         ct = None
         fk = None
diff --git a/tests/generic_relations/tests.py b/tests/generic_relations/tests.py
index 91b1640b57..c22a6ef7e0 100644
--- a/tests/generic_relations/tests.py
+++ b/tests/generic_relations/tests.py
@@ -555,6 +555,23 @@ id="id_generic_relations-taggeditem-content_type-object_id-1-id" /></p>""" % tag
         with self.assertRaises(IntegrityError):
             TaggedItem.objects.create(tag="shiny", content_object=quartz)
 
+    def test_cache_invalidation_for_content_type_id(self):
+        # Create a Vegetable and Mineral with the same id.
+        new_id = max(Vegetable.objects.order_by('-id')[0].id,
+                     Mineral.objects.order_by('-id')[0].id) + 1
+        broccoli = Vegetable.objects.create(id=new_id, name="Broccoli")
+        diamond = Mineral.objects.create(id=new_id, name="Diamond", hardness=7)
+        tag = TaggedItem.objects.create(content_object=broccoli, tag="yummy")
+        tag.content_type = ContentType.objects.get_for_model(diamond)
+        self.assertEqual(tag.content_object, diamond)
+
+    def test_cache_invalidation_for_object_id(self):
+        broccoli = Vegetable.objects.create(name="Broccoli")
+        cauliflower = Vegetable.objects.create(name="Cauliflower")
+        tag = TaggedItem.objects.create(content_object=broccoli, tag="yummy")
+        tag.object_id = cauliflower.id
+        self.assertEqual(tag.content_object, cauliflower)
+
 
 class CustomWidget(forms.TextInput):
     pass