From ee49306176a2d2f1751cb890bd607d42c7c09196 Mon Sep 17 00:00:00 2001
From: Paulo <commonzenpython@gmail.com>
Date: Wed, 20 Sep 2017 23:27:04 -0500
Subject: [PATCH] Fixed #27710 -- Made Model.save() invalidate cached, stale
 relations after a primary key assignment.

---
 django/db/models/base.py   |  4 ++++
 tests/many_to_one/tests.py | 10 ++++++++++
 tests/one_to_one/tests.py  | 10 ++++++++++
 3 files changed, 24 insertions(+)

diff --git a/django/db/models/base.py b/django/db/models/base.py
index e8ef2db913..3f4d65571d 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -674,6 +674,10 @@ class Model(metaclass=ModelBase):
                         "save() prohibited to prevent data loss due to "
                         "unsaved related object '%s'." % field.name
                     )
+                # If the relationship's pk was changed, clear the cached
+                # relationship.
+                if obj and obj.pk != getattr(self, field.attname):
+                    field.delete_cached_value(self)
 
         using = using or router.db_for_write(self.__class__, instance=self)
         if force_insert and (force_update or update_fields):
diff --git a/tests/many_to_one/tests.py b/tests/many_to_one/tests.py
index 0e3395bf3a..a2ff587ab3 100644
--- a/tests/many_to_one/tests.py
+++ b/tests/many_to_one/tests.py
@@ -656,3 +656,13 @@ class ManyToOneTests(TestCase):
         self.assertEqual(city.districts.count(), 2)
         city.districts.remove(d2)
         self.assertEqual(city.districts.count(), 1)
+
+    def test_cached_relation_invalidated_on_save(self):
+        """
+        Model.save() invalidates stale ForeignKey relations after a primary key
+        assignment.
+        """
+        self.assertEqual(self.a.reporter, self.r)  # caches a.reporter
+        self.a.reporter_id = self.r2.pk
+        self.a.save()
+        self.assertEqual(self.a.reporter, self.r2)
diff --git a/tests/one_to_one/tests.py b/tests/one_to_one/tests.py
index 314c56a60f..a3c84a9980 100644
--- a/tests/one_to_one/tests.py
+++ b/tests/one_to_one/tests.py
@@ -507,3 +507,13 @@ class OneToOneTests(TestCase):
         pointer = ToFieldPointer.objects.create(target=target)
         self.assertSequenceEqual(ToFieldPointer.objects.filter(target=target), [pointer])
         self.assertSequenceEqual(ToFieldPointer.objects.filter(pk__exact=pointer), [pointer])
+
+    def test_cached_relation_invalidated_on_save(self):
+        """
+        Model.save() invalidates stale OneToOneField relations after a primary
+        key assignment.
+        """
+        self.assertEqual(self.b1.place, self.p1)  # caches b1.place
+        self.b1.place_id = self.p2.pk
+        self.b1.save()
+        self.assertEqual(self.b1.place, self.p2)