Refs #25546 -- Added detailed comments for prefetch_related test.
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@ -1265,13 +1265,19 @@ class Ticket21760Tests(TestCase):
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self.assertNotIn(' JOIN ', str(queryset.query))
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class Ticket25546Tests(TestCase):
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class DirectPrefechedObjectCacheReuseTests(TestCase):
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"""
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Nested prefetch_related() shouldn't trigger duplicate queries for the same
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lookup.
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prefetch_related() reuses objects fetched in _prefetched_objects_cache.
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Before, prefetch queries were for 'addresses', 'first_time_authors', and
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'first_time_authors__addresses'. The last query is the duplicate.
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When objects are prefetched and not stored as an instance attribute (often
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intermediary relationships), they are saved to the
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_prefetched_objects_cache attribute. prefetch_related() takes
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_prefetched_objects_cache into account when determining whether an object
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has been fetched[1] and retrieves results from it when it is populated [2].
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[1]: #25546 (duplicate queries on nested Prefetch)
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[2]: #27554 (queryset evaluation fails with a mix of nested and flattened
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prefetches)
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"""
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@classmethod
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@ -1291,7 +1297,11 @@ class Ticket25546Tests(TestCase):
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AuthorAddress.objects.create(author=cls.author21, address='Happy place'),
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]
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def test_prefetch(self):
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def test_detect_is_fetched(self):
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"""
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Nested prefetch_related() shouldn't trigger duplicate queries for the same
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lookup.
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"""
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with self.assertNumQueries(3):
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books = Book.objects.filter(
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title__in=['book1', 'book2'],
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@ -1335,7 +1345,7 @@ class Ticket25546Tests(TestCase):
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list(book2.first_time_authors.all()[0].addresses.all().all())
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)
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def test_prefetch_with_to_attr(self):
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def test_detect_is_fetched_with_to_attr(self):
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with self.assertNumQueries(3):
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books = Book.objects.filter(
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title__in=['book1', 'book2'],
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