Fixed #25546 -- Prevented duplicate queries with nested prefetch_related().

django/db/models/query.py

@@ -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

@@ -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])