Fixed #27499 -- Made Prefetches pickle without evaluating their QuerySet.

django/db/models/query.py

@@ -1289,6 +1289,16 @@ class Prefetch(object):
         self.queryset = queryset
         self.to_attr = to_attr
 
+    def __getstate__(self):
+        obj_dict = self.__dict__.copy()
+        if self.queryset is not None:
+            # Prevent the QuerySet from being evaluated
+            obj_dict['queryset'] = self.queryset._clone(
+                _result_cache=[],
+                _prefetch_done=True,
+            )
+        return obj_dict
+
     def add_prefix(self, prefix):
         self.prefetch_through = LOOKUP_SEP.join([prefix, self.prefetch_through])
         self.prefetch_to = LOOKUP_SEP.join([prefix, self.prefetch_to])

tests/queryset_pickle/tests.py

@@ -134,6 +134,15 @@ class PickleabilityTestCase(TestCase):
         groups2 = pickle.loads(pickle.dumps(groups))
         self.assertSequenceEqual(groups2.filter(id__gte=0), [g])
 
+    def test_pickle_prefetch_queryset_not_evaluated(self):
+        Group.objects.create(name='foo')
+        groups = Group.objects.prefetch_related(
+            models.Prefetch('event_set', queryset=Event.objects.order_by('id'))
+        )
+        list(groups)  # evaluate QuerySet
+        with self.assertNumQueries(0):
+            pickle.loads(pickle.dumps(groups))
+
     def test_pickle_prefetch_related_with_m2m_and_objects_deletion(self):
         """
         #24831 -- Cached properties on ManyToOneRel created in QuerySet.delete()