Fixed #18306 -- Made deferred models issue update_fields on save

Deferred models now automatically update only the fields which are
loaded from the db (with .only() or .defer()). In addition, any field
set manually after the load is updated on save.

tests/regressiontests/multiple_database/tests.py

@@ -1546,6 +1546,21 @@ class RouterTestCase(TestCase):
         # If you evaluate the query, it should work, running on 'other'
         self.assertEqual(list(qs.values_list('title', flat=True)), ['Dive into Python'])
 
+    def test_deferred_models(self):
+        mark_def = Person.objects.using('default').create(name="Mark Pilgrim")
+        mark_other = Person.objects.using('other').create(name="Mark Pilgrim")
+        orig_b = Book.objects.using('other').create(title="Dive into Python",
+                                                    published=datetime.date(2009, 5, 4),
+                                                    editor=mark_other)
+        b = Book.objects.using('other').only('title').get(pk=orig_b.pk)
+        self.assertEqual(b.published, datetime.date(2009, 5, 4))
+        b = Book.objects.using('other').only('title').get(pk=orig_b.pk)
+        b.editor = mark_def
+        b.save(using='default')
+        self.assertEqual(Book.objects.using('default').get(pk=b.pk).published,
+                         datetime.date(2009, 5, 4))
+
+
 class AuthTestCase(TestCase):
     multi_db = True