[2.1.x] Fixed #29896 -- Fixed incorrect Model.save() cache relation clearing for foreign keys that use to_field.

Regression in ee49306176.
Backport of f77fc56c96 from master.

django/db/models/base.py

@@ -669,9 +669,9 @@ 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):
+                # If the relationship's pk/to_field was changed, clear the
+                # cached relationship.
+                if obj and getattr(obj, field.target_field.attname) != getattr(self, field.attname):
                     field.delete_cached_value(self)
 
         using = using or router.db_for_write(self.__class__, instance=self)

docs/releases/2.1.3.txt

@@ -17,3 +17,6 @@ Bugfixes
 
 * Fixed a regression in Django 2.0 where test databases aren't reused with
   ``manage.py test --keepdb`` on MySQL (:ticket:`29827`).
+
+* Fixed a regression where cached foreign keys that use ``to_field`` were
+  incorrectly cleared in ``Model.save()`` (:ticket:`29896`).

tests/many_to_one/tests.py

@@ -666,3 +666,9 @@ class ManyToOneTests(TestCase):
         self.a.reporter_id = self.r2.pk
         self.a.save()
         self.assertEqual(self.a.reporter, self.r2)
+
+    def test_cached_foreign_key_with_to_field_not_cleared_by_save(self):
+        parent = Parent.objects.create(name='a')
+        child = ToFieldChild.objects.create(parent=parent)
+        with self.assertNumQueries(0):
+            self.assertIs(child.parent, parent)