Fixed #24873 -- Prevented nested Prefetch objects from being overwritten.

    # contains some prefetch_related lookups. We don't want to trigger the

    # prefetch_related functionality by evaluating the query. Rather, we need

    # to merge in the prefetch_related lookups.

    additional_lookups = getattr(rel_qs, '_prefetch_related_lookups', [])

    # Copy the lookups in case it is a Prefetch object which could be reused

    # later (happens in nested prefetch_related).

    additional_lookups = [

        copy.copy(additional_lookup) for additional_lookup

        in getattr(rel_qs, '_prefetch_related_lookups', [])

    ]

    if additional_lookups:

        # Don't need to clone because the manager should have given us a fresh

        # instance, so we access an internal instead of using public interface

        room = Room.objects.filter(main_room_of__isnull=False).prefetch_related(Prefetch('main_room_of', queryset=houses.filter(address='DoesNotExist'), to_attr='main_room_of_attr')).first()

        self.assertIsNone(room.main_room_of_attr)

    def test_nested_prefetch_related_are_not_overwritten(self):

        # Regression test for #24873

        houses_2 = House.objects.prefetch_related(Prefetch('rooms'))

        persons = Person.objects.prefetch_related(Prefetch('houses', queryset=houses_2))

        houses = House.objects.prefetch_related(Prefetch('occupants', queryset=persons))

        list(houses)  # queryset must be evaluated once to reproduce the bug.

        self.assertEqual(

            houses.all()[0].occupants.all()[0].houses.all()[1].rooms.all()[0],

            self.room2_1

        )

class DefaultManagerTests(TestCase):