[1.8.x] Fixed #24831 -- Fixed pickling queryset with prefetch_related() after deleting objects.

Backport of 2913d6b77d from master

django/db/models/fields/related.py

@@ -1407,6 +1407,11 @@ class ManyToOneRel(ForeignObjectRel):
             parent_link=parent_link, on_delete=on_delete, related_query_name=related_query_name)
         self.field_name = field_name
 
+    def __getstate__(self):
+        state = self.__dict__.copy()
+        state.pop('related_model', None)
+        return state
+
     def get_related_field(self):
         """
         Returns the Field in the 'to' object to which this relationship is

docs/releases/1.8.3.txt

@@ -37,3 +37,6 @@ Bugfixes
 
 * Fixed lack of unique constraint when changing a field from
   ``primary_key=True`` to ``unique=True`` (:ticket:`24893`).
+
+* Fixed queryset pickling when using ``prefetch_related()`` after deleting
+  objects (:ticket:`24831`).

tests/queryset_pickle/tests.py

@@ -120,6 +120,20 @@ class PickleabilityTestCase(TestCase):
         groups = pickle.loads(pickle.dumps(groups))
         self.assertQuerysetEqual(groups, [g], lambda x: x)
 
+    def test_pickle_prefetch_related_with_m2m_and_objects_deletion(self):
+        """
+        #24831 -- Cached properties on ManyToOneRel created in QuerySet.delete()
+        caused subsequent QuerySet pickling to fail.
+        """
+        g = Group.objects.create(name='foo')
+        m2m = M2MModel.objects.create()
+        m2m.groups.add(g)
+        Group.objects.all().delete()
+
+        m2ms = M2MModel.objects.prefetch_related('groups')
+        m2ms = pickle.loads(pickle.dumps(m2ms))
+        self.assertQuerysetEqual(m2ms, [m2m], lambda x: x)
+
     def test_missing_django_version_unpickling(self):
         """
         #21430 -- Verifies a warning is raised for querysets that are