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