Fixed #25546 -- Prevented duplicate queries with nested prefetch_related(). (bdbe50a4) · Commits · Dom Sekotill / django

django/db/models/query.py

+2 −0

Original line number	Diff line number	Diff line
		@@ -1521,6 +1521,7 @@ def get_prefetcher(instance, attr):
		rel_obj = getattr(instance, attr)
		if hasattr(rel_obj, 'get_prefetch_queryset'):
		prefetcher = rel_obj
		is_fetched = attr in instance._prefetched_objects_cache
		return prefetcher, rel_obj_descriptor, attr_found, is_fetched


		@@ -1597,6 +1598,7 @@ def prefetch_one_level(instances, prefetcher, lookup, level):
		else:
		if as_attr:
		setattr(obj, to_attr, vals)
		obj._prefetched_objects_cache[cache_name] = vals
		else:
		# Cache in the QuerySet.all().
		qs = getattr(obj, to_attr).all()

tests/prefetch_related/tests.py

+79 −0

Original line number	Diff line number	Diff line
		@@ -1241,3 +1241,82 @@ class Ticket21760Tests(TestCase):
		prefetcher = get_prefetcher(self.rooms[0], 'house')[0]
		queryset = prefetcher.get_prefetch_queryset(list(Room.objects.all()))[0]
		self.assertNotIn(' JOIN ', force_text(queryset.query))


		class Ticket25546Tests(TestCase):
		"""
		Nested prefetch_related() shouldn't trigger duplicate queries for the same
		lookup.

		Before, prefetch queries were for 'addresses', 'first_time_authors', and
		'first_time_authors__addresses'. The last query is the duplicate.
		"""

		@classmethod
		def setUpTestData(cls):
		cls.book1, cls.book2 = [
		Book.objects.create(title='book1'),
		Book.objects.create(title='book2'),
		]
		cls.author11, cls.author12, cls.author21 = [
		Author.objects.create(first_book=cls.book1, name='Author11'),
		Author.objects.create(first_book=cls.book1, name='Author12'),
		Author.objects.create(first_book=cls.book2, name='Author21'),
		]
		cls.author1_address1, cls.author1_address2, cls.author2_address1 = [
		AuthorAddress.objects.create(author=cls.author11, address='Happy place'),
		AuthorAddress.objects.create(author=cls.author12, address='Haunted house'),
		AuthorAddress.objects.create(author=cls.author21, address='Happy place'),
		]

		def test_prefetch(self):
		with self.assertNumQueries(3):
		books = Book.objects.filter(
		title__in=['book1', 'book2'],
		).prefetch_related(
		Prefetch(
		'first_time_authors',
		Author.objects.prefetch_related(
		Prefetch(
		'addresses',
		AuthorAddress.objects.filter(address='Happy place'),
		)
		),
		),
		)
		book1, book2 = list(books)

		with self.assertNumQueries(0):
		self.assertListEqual(list(book1.first_time_authors.all()), [self.author11, self.author12])
		self.assertListEqual(list(book2.first_time_authors.all()), [self.author21])

		self.assertListEqual(list(book1.first_time_authors.all()[0].addresses.all()), [self.author1_address1])
		self.assertListEqual(list(book1.first_time_authors.all()[1].addresses.all()), [])
		self.assertListEqual(list(book2.first_time_authors.all()[0].addresses.all()), [self.author2_address1])

		def test_prefetch_with_to_attr(self):
		with self.assertNumQueries(3):
		books = Book.objects.filter(
		title__in=['book1', 'book2'],
		).prefetch_related(
		Prefetch(
		'first_time_authors',
		Author.objects.prefetch_related(
		Prefetch(
		'addresses',
		AuthorAddress.objects.filter(address='Happy place'),
		to_attr='happy_place',
		)
		),
		to_attr='first_authors',
		),
		)
		book1, book2 = list(books)

		with self.assertNumQueries(0):
		self.assertListEqual(book1.first_authors, [self.author11, self.author12])
		self.assertListEqual(book2.first_authors, [self.author21])

		self.assertListEqual(book1.first_authors[0].happy_place, [self.author1_address1])
		self.assertListEqual(book1.first_authors[1].happy_place, [])
		self.assertListEqual(book2.first_authors[0].happy_place, [self.author2_address1])