Fixed #27710 -- Made Model.save() invalidate cached, stale relations after a primary key assignment.

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):

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)

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)