[1.2.X] Tweak to many_to_one_null doctest to avoid primary key assumptions (causing breakage on PostgreSQL).

Backport of r14168 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14174 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/modeltests/many_to_one_null/tests.py

@@ -40,7 +40,7 @@ class ManyToOneNullTests(TestCase):
     def test_created_without_related(self):
         self.assertEqual(self.a3.reporter, None)
         # Need to reget a3 to refresh the cache
-        a3 = Article.objects.get(pk=3)
+        a3 = Article.objects.get(pk=self.a3.pk)
         self.assertRaises(AttributeError, getattr, a3.reporter, 'id')
         # Accessing an article's 'reporter' attribute returns None
         # if the reporter is set to None.