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Fixed #32511 -- Corrected handling prefetched nested reverse relationships.
When prefetching a set of child objects related to a set of parent objects, we usually want to populate the relationship back from the child to the parent to avoid a query when accessing that relationship attribute. However, there's an edge case where the child queryset itself specifies a prefetch back to the parent. In that case, we want to use the prefetched relationship rather than populating the reverse relationship from the parent.
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@ -423,6 +423,7 @@ answer newbie questions, and generally made Django that much better:
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James Timmins <jameshtimmins@gmail.com>
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James Turk <dev@jamesturk.net>
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James Wheare <django@sparemint.com>
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Jamie Matthews <jamie@mtth.org>
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Jannis Leidel <jannis@leidel.info>
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Janos Guljas
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Jan Pazdziora
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@ -646,8 +646,9 @@ def create_reverse_many_to_one_manager(superclass, rel):
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# Since we just bypassed this class' get_queryset(), we must manage
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# the reverse relation manually.
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for rel_obj in queryset:
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instance = instances_dict[rel_obj_attr(rel_obj)]
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setattr(rel_obj, self.field.name, instance)
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if not self.field.is_cached(rel_obj):
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instance = instances_dict[rel_obj_attr(rel_obj)]
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setattr(rel_obj, self.field.name, instance)
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cache_name = self.field.remote_field.get_cache_name()
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return queryset, rel_obj_attr, instance_attr, False, cache_name, False
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@ -1614,3 +1614,29 @@ class ReadPrefetchedObjectsCacheTests(TestCase):
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with self.assertNumQueries(4):
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# AuthorWithAge -> Author -> FavoriteAuthors, Book
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self.assertSequenceEqual(authors, [self.author1, self.author2])
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class NestedPrefetchTests(TestCase):
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@classmethod
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def setUpTestData(cls):
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house = House.objects.create(name='Big house', address='123 Main St')
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cls.room = Room.objects.create(name='Kitchen', house=house)
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def test_nested_prefetch_is_not_overwritten_by_related_object(self):
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"""
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The prefetched relationship is used rather than populating the reverse
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relationship from the parent, when prefetching a set of child objects
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related to a set of parent objects and the child queryset itself
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specifies a prefetch back to the parent.
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"""
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queryset = House.objects.only('name').prefetch_related(
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Prefetch('rooms', queryset=Room.objects.prefetch_related(
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Prefetch('house', queryset=House.objects.only('address')),
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)),
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)
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with self.assertNumQueries(3):
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house = queryset.first()
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self.assertIs(Room.house.is_cached(self.room), True)
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with self.assertNumQueries(0):
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house.rooms.first().house.address
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