Fixed #21102 -- pickling a QuerySet with prefetches twice

Fixed the bug that a QuerySet that prefetches related objects cannot be
pickled and unpickled more than once (The second pickling attempt
raises an exception).

Added a new test for the queryset pickling idempotency.

The bug was introduced by
bac187c0d8.

django/db/models/query.py

@@ -80,10 +80,10 @@ class QuerySet(object):
         if model is None:
             # if model is None, then self should be emptyqs and the related
             # objects do not matter.
-            self._known_related_objects = {}
+            obj_dict['_known_related_objects'] = {}
         else:
             opts = model._meta
-            self._known_related_objects = dict(
+            obj_dict['_known_related_objects'] = dict(
                 (opts.get_field(field.name if hasattr(field, 'name') else field), val)
                 for field, val in obj_dict['_known_related_objects'].items()
             )

tests/regressiontests/queryset_pickle/tests.py

@@ -73,3 +73,15 @@ class PickleabilityTestCase(TestCase):
         empty = pickle.loads(dumped)
         self.assertQuerysetEqual(
             empty, [])
+
+    def test_pickle_prefetch_related_idempotence(self):
+        p = Post.objects.create()
+        posts = Post.objects.prefetch_related('materials')
+
+        # First pickling
+        posts = pickle.loads(pickle.dumps(posts))
+        self.assertQuerysetEqual(posts, [p], lambda x: x)
+
+        # Second pickling
+        posts = pickle.loads(pickle.dumps(posts))
+        self.assertQuerysetEqual(posts, [p], lambda x: x)