refresh_from_db() doesn't refresh ForeignObject relations

[Jacob Walls]

Although it's an internal API ForeignObject is now ​quasi-public. When trying it out in a non-composite-pk situation I found that refresh_from_db() didn't clear out a related ForeignObject.

Here is a rough test using the composite_pk models:

tests/composite_pk/test_models.py

@@ -155 +155 @@
         self.assertEqual(4, token.permission_set.count())
         self.assertEqual(4, user.permission_set.count())
         self.assertEqual(4, comment.permission_set.count())
+
+    def test_refresh_foreign_object(self):
+        self.comment_1.user = self.comment_2.user
+        self.assertEqual(self.comment_1.user, self.comment_2.user)
+        self.comment_1.refresh_from_db()
+        self.assertNotEqual(self.comment_1.user, self.comment_2.user)

---

======================================================================
FAIL: test_refresh_foreign_object (composite_pk.test_models.CompositePKModelsTests.test_refresh_foreign_object)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/.../django/tests/composite_pk/test_models.py", line 163, in test_refresh_foreign_object
    self.assertNotEqual(self.comment_1.user, self.comment_2.user)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: <User: User object ((1, 1))> == <User: User object ((1, 1))>

----------------------------------------------------------------------
Ran 1 test in 0.006s

FAILED (failures=1)