django/tests/prefetch_related/tests.py

from unittest import mock

from django.contrib.contenttypes.models import ContentType
from django.core.exceptions import ObjectDoesNotExist
from django.db import NotSupportedError, connection
from django.db.models import Prefetch, QuerySet, prefetch_related_objects
from django.db.models.fields.related import ForwardManyToOneDescriptor
from django.db.models.query import get_prefetcher, prefetch_one_level
from django.db.models.sql import Query
from django.test import (
    TestCase,
    ignore_warnings,
    override_settings,
    skipIfDBFeature,
    skipUnlessDBFeature,
)
from django.test.utils import CaptureQueriesContext
from django.utils.deprecation import RemovedInDjango60Warning

from .models import (
    Article,
    Author,
    Author2,
    AuthorAddress,
    AuthorWithAge,
    Bio,
    Book,
    Bookmark,
    BookReview,
    BookWithYear,
    Comment,
    Department,
    Employee,
    FavoriteAuthors,
    House,
    LessonEntry,
    ModelIterableSubclass,
    Person,
    Qualification,
    Reader,
    Room,
    TaggedItem,
    Teacher,
    WordEntry,
)


class TestDataMixin:
    @classmethod
    def setUpTestData(cls):
        cls.book1 = Book.objects.create(title="Poems")
        cls.book2 = Book.objects.create(title="Jane Eyre")
        cls.book3 = Book.objects.create(title="Wuthering Heights")
        cls.book4 = Book.objects.create(title="Sense and Sensibility")

        cls.author1 = Author.objects.create(name="Charlotte", first_book=cls.book1)
        cls.author2 = Author.objects.create(name="Anne", first_book=cls.book1)
        cls.author3 = Author.objects.create(name="Emily", first_book=cls.book1)
        cls.author4 = Author.objects.create(name="Jane", first_book=cls.book4)

        cls.book1.authors.add(cls.author1, cls.author2, cls.author3)
        cls.book2.authors.add(cls.author1)
        cls.book3.authors.add(cls.author3)
        cls.book4.authors.add(cls.author4)

        cls.reader1 = Reader.objects.create(name="Amy")
        cls.reader2 = Reader.objects.create(name="Belinda")

        cls.reader1.books_read.add(cls.book1, cls.book4)
        cls.reader2.books_read.add(cls.book2, cls.book4)


class PrefetchRelatedTests(TestDataMixin, TestCase):
    def assertWhereContains(self, sql, needle):
        where_idx = sql.index("WHERE")
        self.assertEqual(
            sql.count(str(needle), where_idx),
            1,
            msg="WHERE clause doesn't contain %s, actual SQL: %s"
            % (needle, sql[where_idx:]),
        )

    def test_m2m_forward(self):
        with self.assertNumQueries(2):
            lists = [
                list(b.authors.all()) for b in Book.objects.prefetch_related("authors")
            ]

        normal_lists = [list(b.authors.all()) for b in Book.objects.all()]
        self.assertEqual(lists, normal_lists)

    def test_m2m_reverse(self):
        with self.assertNumQueries(2):
            lists = [
                list(a.books.all()) for a in Author.objects.prefetch_related("books")
            ]

        normal_lists = [list(a.books.all()) for a in Author.objects.all()]
        self.assertEqual(lists, normal_lists)

    def test_foreignkey_forward(self):
        with self.assertNumQueries(2):
            books = [
                a.first_book for a in Author.objects.prefetch_related("first_book")
            ]

        normal_books = [a.first_book for a in Author.objects.all()]
        self.assertEqual(books, normal_books)

    def test_foreignkey_reverse(self):
        with self.assertNumQueries(2):
            [
                list(b.first_time_authors.all())
                for b in Book.objects.prefetch_related("first_time_authors")
            ]

        self.assertSequenceEqual(self.book2.authors.all(), [self.author1])

    def test_onetoone_reverse_no_match(self):
        # Regression for #17439
        with self.assertNumQueries(2):
            book = Book.objects.prefetch_related("bookwithyear").all()[0]
        with self.assertNumQueries(0):
            with self.assertRaises(BookWithYear.DoesNotExist):
                book.bookwithyear

    def test_onetoone_reverse_with_to_field_pk(self):
        """
        A model (Bio) with a OneToOneField primary key (author) that references
        a non-pk field (name) on the related model (Author) is prefetchable.
        """
        Bio.objects.bulk_create(
            [
                Bio(author=self.author1),
                Bio(author=self.author2),
                Bio(author=self.author3),
            ]
        )
        authors = Author.objects.filter(
            name__in=[self.author1, self.author2, self.author3],
        ).prefetch_related("bio")
        with self.assertNumQueries(2):
            for author in authors:
                self.assertEqual(author.name, author.bio.author.name)

    def test_survives_clone(self):
        with self.assertNumQueries(2):
            [
                list(b.first_time_authors.all())
                for b in Book.objects.prefetch_related("first_time_authors").exclude(
                    id=1000
                )
            ]

    def test_len(self):
        with self.assertNumQueries(2):
            qs = Book.objects.prefetch_related("first_time_authors")
            len(qs)
            [list(b.first_time_authors.all()) for b in qs]

    def test_bool(self):
        with self.assertNumQueries(2):
            qs = Book.objects.prefetch_related("first_time_authors")
            bool(qs)
            [list(b.first_time_authors.all()) for b in qs]

    def test_count(self):
        with self.assertNumQueries(2):
            qs = Book.objects.prefetch_related("first_time_authors")
            [b.first_time_authors.count() for b in qs]

    def test_exists(self):
        with self.assertNumQueries(2):
            qs = Book.objects.prefetch_related("first_time_authors")
            [b.first_time_authors.exists() for b in qs]

    def test_in_and_prefetch_related(self):
        """
        Regression test for #20242 - QuerySet "in" didn't work the first time
        when using prefetch_related. This was fixed by the removal of chunked
        reads from QuerySet iteration in
        70679243d1786e03557c28929f9762a119e3ac14.
        """
        qs = Book.objects.prefetch_related("first_time_authors")
        self.assertIn(qs[0], qs)

    def test_clear(self):
        with self.assertNumQueries(5):
            with_prefetch = Author.objects.prefetch_related("books")
            without_prefetch = with_prefetch.prefetch_related(None)
            [list(a.books.all()) for a in without_prefetch]

    def test_m2m_then_m2m(self):
        """A m2m can be followed through another m2m."""
        with self.assertNumQueries(3):
            qs = Author.objects.prefetch_related("books__read_by")
            lists = [
                [[str(r) for r in b.read_by.all()] for b in a.books.all()] for a in qs
            ]
            self.assertEqual(
                lists,
                [
                    [["Amy"], ["Belinda"]],  # Charlotte - Poems, Jane Eyre
                    [["Amy"]],  # Anne - Poems
                    [["Amy"], []],  # Emily - Poems, Wuthering Heights
                    [["Amy", "Belinda"]],  # Jane - Sense and Sense
                ],
            )

    def test_overriding_prefetch(self):
        with self.assertNumQueries(3):
            qs = Author.objects.prefetch_related("books", "books__read_by")
            lists = [
                [[str(r) for r in b.read_by.all()] for b in a.books.all()] for a in qs
            ]
            self.assertEqual(
                lists,
                [
                    [["Amy"], ["Belinda"]],  # Charlotte - Poems, Jane Eyre
                    [["Amy"]],  # Anne - Poems
                    [["Amy"], []],  # Emily - Poems, Wuthering Heights
                    [["Amy", "Belinda"]],  # Jane - Sense and Sense
                ],
            )
        with self.assertNumQueries(3):
            qs = Author.objects.prefetch_related("books__read_by", "books")
            lists = [
                [[str(r) for r in b.read_by.all()] for b in a.books.all()] for a in qs
            ]
            self.assertEqual(
                lists,
                [
                    [["Amy"], ["Belinda"]],  # Charlotte - Poems, Jane Eyre
                    [["Amy"]],  # Anne - Poems
                    [["Amy"], []],  # Emily - Poems, Wuthering Heights
                    [["Amy", "Belinda"]],  # Jane - Sense and Sense
                ],
            )

    def test_get(self):
        """
        Objects retrieved with .get() get the prefetch behavior.
        """
        # Need a double
        with self.assertNumQueries(3):
            author = Author.objects.prefetch_related("books__read_by").get(
                name="Charlotte"
            )
            lists = [[str(r) for r in b.read_by.all()] for b in author.books.all()]
            self.assertEqual(lists, [["Amy"], ["Belinda"]])  # Poems, Jane Eyre

    def test_foreign_key_then_m2m(self):
        """
        A m2m relation can be followed after a relation like ForeignKey that
        doesn't have many objects.
        """
        with self.assertNumQueries(2):
            qs = Author.objects.select_related("first_book").prefetch_related(
                "first_book__read_by"
            )
            lists = [[str(r) for r in a.first_book.read_by.all()] for a in qs]
            self.assertEqual(lists, [["Amy"], ["Amy"], ["Amy"], ["Amy", "Belinda"]])

    def test_reverse_one_to_one_then_m2m(self):
        """
        A m2m relation can be followed after going through the select_related
        reverse of an o2o.
        """
        qs = Author.objects.prefetch_related("bio__books").select_related("bio")

        with self.assertNumQueries(1):
            list(qs.all())

        Bio.objects.create(author=self.author1)
        with self.assertNumQueries(2):
            list(qs.all())

    def test_attribute_error(self):
        qs = Reader.objects.prefetch_related("books_read__xyz")
        msg = (
            "Cannot find 'xyz' on Book object, 'books_read__xyz' "
            "is an invalid parameter to prefetch_related()"
        )
        with self.assertRaisesMessage(AttributeError, msg) as cm:
            list(qs)

        self.assertIn("prefetch_related", str(cm.exception))

    def test_invalid_final_lookup(self):
        qs = Book.objects.prefetch_related("authors__name")
        msg = (
            "'authors__name' does not resolve to an item that supports "
            "prefetching - this is an invalid parameter to prefetch_related()."
        )
        with self.assertRaisesMessage(ValueError, msg) as cm:
            list(qs)

        self.assertIn("prefetch_related", str(cm.exception))
        self.assertIn("name", str(cm.exception))

    def test_prefetch_eq(self):
        prefetch_1 = Prefetch("authors", queryset=Author.objects.all())
        prefetch_2 = Prefetch("books", queryset=Book.objects.all())
        self.assertEqual(prefetch_1, prefetch_1)
        self.assertEqual(prefetch_1, mock.ANY)
        self.assertNotEqual(prefetch_1, prefetch_2)

    def test_forward_m2m_to_attr_conflict(self):
        msg = "to_attr=authors conflicts with a field on the Book model."
        authors = Author.objects.all()
        with self.assertRaisesMessage(ValueError, msg):
            list(
                Book.objects.prefetch_related(
                    Prefetch("authors", queryset=authors, to_attr="authors"),
                )
            )
        # Without the ValueError, an author was deleted due to the implicit
        # save of the relation assignment.
        self.assertEqual(self.book1.authors.count(), 3)

    def test_reverse_m2m_to_attr_conflict(self):
        msg = "to_attr=books conflicts with a field on the Author model."
        poems = Book.objects.filter(title="Poems")
        with self.assertRaisesMessage(ValueError, msg):
            list(
                Author.objects.prefetch_related(
                    Prefetch("books", queryset=poems, to_attr="books"),
                )
            )
        # Without the ValueError, a book was deleted due to the implicit
        # save of reverse relation assignment.
        self.assertEqual(self.author1.books.count(), 2)

    def test_m2m_then_reverse_fk_object_ids(self):
        with CaptureQueriesContext(connection) as queries:
            list(Book.objects.prefetch_related("authors__addresses"))

        sql = queries[-1]["sql"]
        self.assertWhereContains(sql, self.author1.name)

    def test_m2m_then_m2m_object_ids(self):
        with CaptureQueriesContext(connection) as queries:
            list(Book.objects.prefetch_related("authors__favorite_authors"))

        sql = queries[-1]["sql"]
        self.assertWhereContains(sql, self.author1.name)

    def test_m2m_then_reverse_one_to_one_object_ids(self):
        with CaptureQueriesContext(connection) as queries:
            list(Book.objects.prefetch_related("authors__authorwithage"))

        sql = queries[-1]["sql"]
        self.assertWhereContains(sql, self.author1.id)

    def test_filter_deferred(self):
        """
        Related filtering of prefetched querysets is deferred on m2m and
        reverse m2o relations until necessary.
        """
        add_q = Query.add_q
        for relation in ["authors", "first_time_authors"]:
            with self.subTest(relation=relation):
                with mock.patch.object(
                    Query,
                    "add_q",
                    autospec=True,
                    side_effect=lambda self, q: add_q(self, q),
                ) as add_q_mock:
                    list(Book.objects.prefetch_related(relation))
                    self.assertEqual(add_q_mock.call_count, 1)

    def test_named_values_list(self):
        qs = Author.objects.prefetch_related("books")
        self.assertCountEqual(
            [value.name for value in qs.values_list("name", named=True)],
            ["Anne", "Charlotte", "Emily", "Jane"],
        )

    def test_m2m_prefetching_iterator_with_chunks(self):
        with self.assertNumQueries(3):
            authors = [
                b.authors.first()
                for b in Book.objects.prefetch_related("authors").iterator(chunk_size=2)
            ]
        self.assertEqual(
            authors,
            [self.author1, self.author1, self.author3, self.author4],
        )

    def test_m2m_prefetching_iterator_without_chunks_error(self):
        msg = (
            "chunk_size must be provided when using QuerySet.iterator() after "
            "prefetch_related()."
        )
        with self.assertRaisesMessage(ValueError, msg):
            Book.objects.prefetch_related("authors").iterator()


class RawQuerySetTests(TestDataMixin, TestCase):
    def test_basic(self):
        with self.assertNumQueries(2):
            books = Book.objects.raw(
                "SELECT * FROM prefetch_related_book WHERE id = %s", (self.book1.id,)
            ).prefetch_related("authors")
            book1 = list(books)[0]

        with self.assertNumQueries(0):
            self.assertCountEqual(
                book1.authors.all(), [self.author1, self.author2, self.author3]
            )

    def test_prefetch_before_raw(self):
        with self.assertNumQueries(2):
            books = Book.objects.prefetch_related("authors").raw(
                "SELECT * FROM prefetch_related_book WHERE id = %s", (self.book1.id,)
            )
            book1 = list(books)[0]

        with self.assertNumQueries(0):
            self.assertCountEqual(
                book1.authors.all(), [self.author1, self.author2, self.author3]
            )

    def test_clear(self):
        with self.assertNumQueries(5):
            with_prefetch = Author.objects.raw(
                "SELECT * FROM prefetch_related_author"
            ).prefetch_related("books")
            without_prefetch = with_prefetch.prefetch_related(None)
            [list(a.books.all()) for a in without_prefetch]


class CustomPrefetchTests(TestCase):
    @classmethod
    def traverse_qs(cls, obj_iter, path):
        """
        Helper method that returns a list containing a list of the objects in the
        obj_iter. Then for each object in the obj_iter, the path will be
        recursively travelled and the found objects are added to the return value.
        """
        ret_val = []

        if hasattr(obj_iter, "all"):
            obj_iter = obj_iter.all()

        try:
            iter(obj_iter)
        except TypeError:
            obj_iter = [obj_iter]

        for obj in obj_iter:
            rel_objs = []
            for part in path:
                if not part:
                    continue
                try:
                    related = getattr(obj, part[0])
                except ObjectDoesNotExist:
                    continue
                if related is not None:
                    rel_objs.extend(cls.traverse_qs(related, [part[1:]]))
            ret_val.append((obj, rel_objs))
        return ret_val

    @classmethod
    def setUpTestData(cls):
        cls.person1 = Person.objects.create(name="Joe")
        cls.person2 = Person.objects.create(name="Mary")

        # Set main_room for each house before creating the next one for
        # databases where supports_nullable_unique_constraints is False.

        cls.house1 = House.objects.create(
            name="House 1", address="123 Main St", owner=cls.person1
        )
        cls.room1_1 = Room.objects.create(name="Dining room", house=cls.house1)
        cls.room1_2 = Room.objects.create(name="Lounge", house=cls.house1)
        cls.room1_3 = Room.objects.create(name="Kitchen", house=cls.house1)
        cls.house1.main_room = cls.room1_1
        cls.house1.save()
        cls.person1.houses.add(cls.house1)

        cls.house2 = House.objects.create(
            name="House 2", address="45 Side St", owner=cls.person1
        )
        cls.room2_1 = Room.objects.create(name="Dining room", house=cls.house2)
        cls.room2_2 = Room.objects.create(name="Lounge", house=cls.house2)
        cls.room2_3 = Room.objects.create(name="Kitchen", house=cls.house2)
        cls.house2.main_room = cls.room2_1
        cls.house2.save()
        cls.person1.houses.add(cls.house2)

        cls.house3 = House.objects.create(
            name="House 3", address="6 Downing St", owner=cls.person2
        )
        cls.room3_1 = Room.objects.create(name="Dining room", house=cls.house3)
        cls.room3_2 = Room.objects.create(name="Lounge", house=cls.house3)
        cls.room3_3 = Room.objects.create(name="Kitchen", house=cls.house3)
        cls.house3.main_room = cls.room3_1
        cls.house3.save()
        cls.person2.houses.add(cls.house3)

        cls.house4 = House.objects.create(
            name="house 4", address="7 Regents St", owner=cls.person2
        )
        cls.room4_1 = Room.objects.create(name="Dining room", house=cls.house4)
        cls.room4_2 = Room.objects.create(name="Lounge", house=cls.house4)
        cls.room4_3 = Room.objects.create(name="Kitchen", house=cls.house4)
        cls.house4.main_room = cls.room4_1
        cls.house4.save()
        cls.person2.houses.add(cls.house4)

    def test_traverse_qs(self):
        qs = Person.objects.prefetch_related("houses")
        related_objs_normal = ([list(p.houses.all()) for p in qs],)
        related_objs_from_traverse = [
            [inner[0] for inner in o[1]] for o in self.traverse_qs(qs, [["houses"]])
        ]
        self.assertEqual(related_objs_normal, (related_objs_from_traverse,))

    def test_ambiguous(self):
        # Ambiguous: Lookup was already seen with a different queryset.
        msg = (
            "'houses' lookup was already seen with a different queryset. You "
            "may need to adjust the ordering of your lookups."
        )
        # lookup.queryset shouldn't be evaluated.
        with self.assertNumQueries(3):
            with self.assertRaisesMessage(ValueError, msg):
                self.traverse_qs(
                    Person.objects.prefetch_related(
                        "houses__rooms",
                        Prefetch("houses", queryset=House.objects.all()),
                    ),
                    [["houses", "rooms"]],
                )

        # Ambiguous: Lookup houses_lst doesn't yet exist when performing
        # houses_lst__rooms.
        msg = (
            "Cannot find 'houses_lst' on Person object, 'houses_lst__rooms' is "
            "an invalid parameter to prefetch_related()"
        )
        with self.assertRaisesMessage(AttributeError, msg):
            self.traverse_qs(
                Person.objects.prefetch_related(
                    "houses_lst__rooms",
                    Prefetch(
                        "houses", queryset=House.objects.all(), to_attr="houses_lst"
                    ),
                ),
                [["houses", "rooms"]],
            )

        # Not ambiguous.
        self.traverse_qs(
            Person.objects.prefetch_related("houses__rooms", "houses"),
            [["houses", "rooms"]],
        )

        self.traverse_qs(
            Person.objects.prefetch_related(
                "houses__rooms",
                Prefetch("houses", queryset=House.objects.all(), to_attr="houses_lst"),
            ),
            [["houses", "rooms"]],
        )

    def test_m2m(self):
        # Control lookups.
        with self.assertNumQueries(2):
            lst1 = self.traverse_qs(
                Person.objects.prefetch_related("houses"), [["houses"]]
            )

        # Test lookups.
        with self.assertNumQueries(2):
            lst2 = self.traverse_qs(
                Person.objects.prefetch_related(Prefetch("houses")), [["houses"]]
            )
        self.assertEqual(lst1, lst2)
        with self.assertNumQueries(2):
            lst2 = self.traverse_qs(
                Person.objects.prefetch_related(
                    Prefetch("houses", to_attr="houses_lst")
                ),
                [["houses_lst"]],
            )
        self.assertEqual(lst1, lst2)

    def test_reverse_m2m(self):
        # Control lookups.
        with self.assertNumQueries(2):
            lst1 = self.traverse_qs(
                House.objects.prefetch_related("occupants"), [["occupants"]]
            )

        # Test lookups.
        with self.assertNumQueries(2):
            lst2 = self.traverse_qs(
                House.objects.prefetch_related(Prefetch("occupants")), [["occupants"]]
            )
        self.assertEqual(lst1, lst2)
        with self.assertNumQueries(2):
            lst2 = self.traverse_qs(
                House.objects.prefetch_related(
                    Prefetch("occupants", to_attr="occupants_lst")
                ),
                [["occupants_lst"]],
            )
        self.assertEqual(lst1, lst2)

    def test_m2m_through_fk(self):
        # Control lookups.
        with self.assertNumQueries(3):
            lst1 = self.traverse_qs(
                Room.objects.prefetch_related("house__occupants"),
                [["house", "occupants"]],
            )

        # Test lookups.
        with self.assertNumQueries(3):
            lst2 = self.traverse_qs(
                Room.objects.prefetch_related(Prefetch("house__occupants")),
                [["house", "occupants"]],
            )
        self.assertEqual(lst1, lst2)
        with self.assertNumQueries(3):
            lst2 = self.traverse_qs(
                Room.objects.prefetch_related(
                    Prefetch("house__occupants", to_attr="occupants_lst")
                ),
                [["house", "occupants_lst"]],
            )
        self.assertEqual(lst1, lst2)

    def test_m2m_through_gfk(self):
        TaggedItem.objects.create(tag="houses", content_object=self.house1)
        TaggedItem.objects.create(tag="houses", content_object=self.house2)

        # Control lookups.
        with self.assertNumQueries(3):
            lst1 = self.traverse_qs(
                TaggedItem.objects.filter(tag="houses").prefetch_related(
                    "content_object__rooms"
                ),
                [["content_object", "rooms"]],
            )

        # Test lookups.
        with self.assertNumQueries(3):
            lst2 = self.traverse_qs(
                TaggedItem.objects.prefetch_related(
                    Prefetch("content_object"),
                    Prefetch("content_object__rooms", to_attr="rooms_lst"),
                ),
                [["content_object", "rooms_lst"]],
            )
        self.assertEqual(lst1, lst2)

    def test_o2m_through_m2m(self):
        # Control lookups.
        with self.assertNumQueries(3):
            lst1 = self.traverse_qs(
                Person.objects.prefetch_related("houses", "houses__rooms"),
                [["houses", "rooms"]],
            )

        # Test lookups.
        with self.assertNumQueries(3):
            lst2 = self.traverse_qs(
                Person.objects.prefetch_related(Prefetch("houses"), "houses__rooms"),
                [["houses", "rooms"]],
            )
        self.assertEqual(lst1, lst2)
        with self.assertNumQueries(3):
            lst2 = self.traverse_qs(
                Person.objects.prefetch_related(
                    Prefetch("houses"), Prefetch("houses__rooms")
                ),
                [["houses", "rooms"]],
            )
        self.assertEqual(lst1, lst2)
        with self.assertNumQueries(3):
            lst2 = self.traverse_qs(
                Person.objects.prefetch_related(
                    Prefetch("houses", to_attr="houses_lst"), "houses_lst__rooms"
                ),
                [["houses_lst", "rooms"]],
            )
        self.assertEqual(lst1, lst2)
        with self.assertNumQueries(3):
            lst2 = self.traverse_qs(
                Person.objects.prefetch_related(
                    Prefetch("houses", to_attr="houses_lst"),
                    Prefetch("houses_lst__rooms", to_attr="rooms_lst"),
                ),
                [["houses_lst", "rooms_lst"]],
            )
        self.assertEqual(lst1, lst2)

    def test_generic_rel(self):
        bookmark = Bookmark.objects.create(url="http://www.djangoproject.com/")
        TaggedItem.objects.create(content_object=bookmark, tag="django")
        TaggedItem.objects.create(
            content_object=bookmark, favorite=bookmark, tag="python"
        )

        # Control lookups.
        with self.assertNumQueries(4):
            lst1 = self.traverse_qs(
                Bookmark.objects.prefetch_related(
                    "tags", "tags__content_object", "favorite_tags"
                ),
                [["tags", "content_object"], ["favorite_tags"]],
            )

        # Test lookups.
        with self.assertNumQueries(4):
            lst2 = self.traverse_qs(
                Bookmark.objects.prefetch_related(
                    Prefetch("tags", to_attr="tags_lst"),
                    Prefetch("tags_lst__content_object"),
                    Prefetch("favorite_tags"),
                ),
                [["tags_lst", "content_object"], ["favorite_tags"]],
            )
        self.assertEqual(lst1, lst2)

    def test_traverse_single_item_property(self):
        # Control lookups.
        with self.assertNumQueries(5):
            lst1 = self.traverse_qs(
                Person.objects.prefetch_related(
                    "houses__rooms",
                    "primary_house__occupants__houses",
                ),
                [["primary_house", "occupants", "houses"]],
            )

        # Test lookups.
        with self.assertNumQueries(5):
            lst2 = self.traverse_qs(
                Person.objects.prefetch_related(
                    "houses__rooms",
                    Prefetch("primary_house__occupants", to_attr="occupants_lst"),
                    "primary_house__occupants_lst__houses",
                ),
                [["primary_house", "occupants_lst", "houses"]],
            )
        self.assertEqual(lst1, lst2)

    def test_traverse_multiple_items_property(self):
        # Control lookups.
        with self.assertNumQueries(4):
            lst1 = self.traverse_qs(
                Person.objects.prefetch_related(
                    "houses",
                    "all_houses__occupants__houses",
                ),
                [["all_houses", "occupants", "houses"]],
            )

        # Test lookups.
        with self.assertNumQueries(4):
            lst2 = self.traverse_qs(
                Person.objects.prefetch_related(
                    "houses",
                    Prefetch("all_houses__occupants", to_attr="occupants_lst"),
                    "all_houses__occupants_lst__houses",
                ),
                [["all_houses", "occupants_lst", "houses"]],
            )
        self.assertEqual(lst1, lst2)

    def test_custom_qs(self):
        # Test basic.
        with self.assertNumQueries(2):
            lst1 = list(Person.objects.prefetch_related("houses"))
        with self.assertNumQueries(2):
            lst2 = list(
                Person.objects.prefetch_related(
                    Prefetch(
                        "houses", queryset=House.objects.all(), to_attr="houses_lst"
                    )
                )
            )
        self.assertEqual(
            self.traverse_qs(lst1, [["houses"]]),
            self.traverse_qs(lst2, [["houses_lst"]]),
        )

        # Test queryset filtering.
        with self.assertNumQueries(2):
            lst2 = list(
                Person.objects.prefetch_related(
                    Prefetch(
                        "houses",
                        queryset=House.objects.filter(
                            pk__in=[self.house1.pk, self.house3.pk]
                        ),
                        to_attr="houses_lst",
                    )
                )
            )
        self.assertEqual(len(lst2[0].houses_lst), 1)
        self.assertEqual(lst2[0].houses_lst[0], self.house1)
        self.assertEqual(len(lst2[1].houses_lst), 1)
        self.assertEqual(lst2[1].houses_lst[0], self.house3)

        # Test flattened.
        with self.assertNumQueries(3):
            lst1 = list(Person.objects.prefetch_related("houses__rooms"))
        with self.assertNumQueries(3):
            lst2 = list(
                Person.objects.prefetch_related(
                    Prefetch(
                        "houses__rooms",
                        queryset=Room.objects.all(),
                        to_attr="rooms_lst",
                    )
                )
            )
        self.assertEqual(
            self.traverse_qs(lst1, [["houses", "rooms"]]),
            self.traverse_qs(lst2, [["houses", "rooms_lst"]]),
        )

        # Test inner select_related.
        with self.assertNumQueries(3):
            lst1 = list(Person.objects.prefetch_related("houses__owner"))
        with self.assertNumQueries(2):
            lst2 = list(
                Person.objects.prefetch_related(
                    Prefetch("houses", queryset=House.objects.select_related("owner"))
                )
            )
        self.assertEqual(
            self.traverse_qs(lst1, [["houses", "owner"]]),
            self.traverse_qs(lst2, [["houses", "owner"]]),
        )

        # Test inner prefetch.
        inner_rooms_qs = Room.objects.filter(pk__in=[self.room1_1.pk, self.room1_2.pk])
        houses_qs_prf = House.objects.prefetch_related(
            Prefetch("rooms", queryset=inner_rooms_qs, to_attr="rooms_lst")
        )
        with self.assertNumQueries(4):
            lst2 = list(
                Person.objects.prefetch_related(
                    Prefetch(
                        "houses",
                        queryset=houses_qs_prf.filter(pk=self.house1.pk),
                        to_attr="houses_lst",
                    ),
                    Prefetch("houses_lst__rooms_lst__main_room_of"),
                )
            )

        self.assertEqual(len(lst2[0].houses_lst[0].rooms_lst), 2)
        self.assertEqual(lst2[0].houses_lst[0].rooms_lst[0], self.room1_1)
        self.assertEqual(lst2[0].houses_lst[0].rooms_lst[1], self.room1_2)
        self.assertEqual(lst2[0].houses_lst[0].rooms_lst[0].main_room_of, self.house1)
        self.assertEqual(len(lst2[1].houses_lst), 0)

        # Test ForwardManyToOneDescriptor.
        houses = House.objects.select_related("owner")
        with self.assertNumQueries(6):
            rooms = Room.objects.prefetch_related("house")
            lst1 = self.traverse_qs(rooms, [["house", "owner"]])
        with self.assertNumQueries(2):
            rooms = Room.objects.prefetch_related(Prefetch("house", queryset=houses))
            lst2 = self.traverse_qs(rooms, [["house", "owner"]])
        self.assertEqual(lst1, lst2)
        with self.assertNumQueries(2):
            houses = House.objects.select_related("owner")
            rooms = Room.objects.prefetch_related(
                Prefetch("house", queryset=houses, to_attr="house_attr")
            )
            lst2 = self.traverse_qs(rooms, [["house_attr", "owner"]])
        self.assertEqual(lst1, lst2)
        room = Room.objects.prefetch_related(
            Prefetch("house", queryset=houses.filter(address="DoesNotExist"))
        ).first()
        with self.assertRaises(ObjectDoesNotExist):
            getattr(room, "house")
        room = Room.objects.prefetch_related(
            Prefetch(
                "house",
                queryset=houses.filter(address="DoesNotExist"),
                to_attr="house_attr",
            )
        ).first()
        self.assertIsNone(room.house_attr)
        rooms = Room.objects.prefetch_related(
            Prefetch("house", queryset=House.objects.only("name"))
        )
        with self.assertNumQueries(2):
            getattr(rooms.first().house, "name")
        with self.assertNumQueries(3):
            getattr(rooms.first().house, "address")

        # Test ReverseOneToOneDescriptor.
        houses = House.objects.select_related("owner")
        with self.assertNumQueries(6):
            rooms = Room.objects.prefetch_related("main_room_of")
            lst1 = self.traverse_qs(rooms, [["main_room_of", "owner"]])
        with self.assertNumQueries(2):
            rooms = Room.objects.prefetch_related(
                Prefetch("main_room_of", queryset=houses)
            )
            lst2 = self.traverse_qs(rooms, [["main_room_of", "owner"]])
        self.assertEqual(lst1, lst2)
        with self.assertNumQueries(2):
            rooms = list(
                Room.objects.prefetch_related(
                    Prefetch(
                        "main_room_of",
                        queryset=houses,
                        to_attr="main_room_of_attr",
                    )
                )
            )
            lst2 = self.traverse_qs(rooms, [["main_room_of_attr", "owner"]])
        self.assertEqual(lst1, lst2)
        room = (
            Room.objects.filter(main_room_of__isnull=False)
            .prefetch_related(
                Prefetch("main_room_of", queryset=houses.filter(address="DoesNotExist"))
            )
            .first()
        )
        with self.assertRaises(ObjectDoesNotExist):
            getattr(room, "main_room_of")
        room = (
            Room.objects.filter(main_room_of__isnull=False)
            .prefetch_related(
                Prefetch(
                    "main_room_of",
                    queryset=houses.filter(address="DoesNotExist"),
                    to_attr="main_room_of_attr",
                )
            )
            .first()
        )
        self.assertIsNone(room.main_room_of_attr)

        # The custom queryset filters should be applied to the queryset
        # instance returned by the manager.
        person = Person.objects.prefetch_related(
            Prefetch("houses", queryset=House.objects.filter(name="House 1")),
        ).get(pk=self.person1.pk)
        self.assertEqual(
            list(person.houses.all()),
            list(person.houses.all().all()),
        )

    def test_nested_prefetch_related_are_not_overwritten(self):
        # Regression test for #24873
        houses_2 = House.objects.prefetch_related(Prefetch("rooms"))
        persons = Person.objects.prefetch_related(Prefetch("houses", queryset=houses_2))
        houses = House.objects.prefetch_related(Prefetch("occupants", queryset=persons))
        list(houses)  # queryset must be evaluated once to reproduce the bug.
        self.assertEqual(
            houses.all()[0].occupants.all()[0].houses.all()[1].rooms.all()[0],
            self.room2_1,
        )

    def test_nested_prefetch_related_with_duplicate_prefetcher(self):
        """
        Nested prefetches whose name clashes with descriptor names
        (Person.houses here) are allowed.
        """
        occupants = Person.objects.prefetch_related(
            Prefetch("houses", to_attr="some_attr_name"),
            Prefetch("houses", queryset=House.objects.prefetch_related("main_room")),
        )
        houses = House.objects.prefetch_related(
            Prefetch("occupants", queryset=occupants)
        )
        with self.assertNumQueries(5):
            self.traverse_qs(list(houses), [["occupants", "houses", "main_room"]])

    def test_nested_prefetch_related_with_duplicate_prefetch_and_depth(self):
        people = Person.objects.prefetch_related(
            Prefetch(
                "houses__main_room",
                queryset=Room.objects.filter(name="Dining room"),
                to_attr="dining_room",
            ),
            "houses__main_room",
        )
        with self.assertNumQueries(4):
            main_room = people[0].houses.all()[0]

        people = Person.objects.prefetch_related(
            "houses__main_room",
            Prefetch(
                "houses__main_room",
                queryset=Room.objects.filter(name="Dining room"),
                to_attr="dining_room",
            ),
        )
        with self.assertNumQueries(4):
            main_room = people[0].houses.all()[0]

        self.assertEqual(main_room.main_room, self.room1_1)

    def test_values_queryset(self):
        msg = "Prefetch querysets cannot use raw(), values(), and values_list()."
        with self.assertRaisesMessage(ValueError, msg):
            Prefetch("houses", House.objects.values("pk"))
        with self.assertRaisesMessage(ValueError, msg):
            Prefetch("houses", House.objects.values_list("pk"))
        # That error doesn't affect managers with custom ModelIterable subclasses
        self.assertIs(
            Teacher.objects_custom.all()._iterable_class, ModelIterableSubclass
        )
        Prefetch("teachers", Teacher.objects_custom.all())

    def test_raw_queryset(self):
        msg = "Prefetch querysets cannot use raw(), values(), and values_list()."
        with self.assertRaisesMessage(ValueError, msg):
            Prefetch("houses", House.objects.raw("select pk from house"))

    def test_to_attr_doesnt_cache_through_attr_as_list(self):
        house = House.objects.prefetch_related(
            Prefetch("rooms", queryset=Room.objects.all(), to_attr="to_rooms"),
        ).get(pk=self.house3.pk)
        self.assertIsInstance(house.rooms.all(), QuerySet)

    def test_to_attr_cached_property(self):
        persons = Person.objects.prefetch_related(
            Prefetch("houses", House.objects.all(), to_attr="cached_all_houses"),
        )
        for person in persons:
            # To bypass caching at the related descriptor level, don't use
            # person.houses.all() here.
            all_houses = list(House.objects.filter(occupants=person))
            with self.assertNumQueries(0):
                self.assertEqual(person.cached_all_houses, all_houses)

    def test_filter_deferred(self):
        """
        Related filtering of prefetched querysets is deferred until necessary.
        """
        add_q = Query.add_q
        with mock.patch.object(
            Query,
            "add_q",
            autospec=True,
            side_effect=lambda self, q: add_q(self, q),
        ) as add_q_mock:
            list(
                House.objects.prefetch_related(
                    Prefetch("occupants", queryset=Person.objects.all())
                )
            )
            self.assertEqual(add_q_mock.call_count, 1)


class DefaultManagerTests(TestCase):
    @classmethod
    def setUpTestData(cls):
        cls.qual1 = Qualification.objects.create(name="BA")
        cls.qual2 = Qualification.objects.create(name="BSci")
        cls.qual3 = Qualification.objects.create(name="MA")
        cls.qual4 = Qualification.objects.create(name="PhD")

        cls.teacher1 = Teacher.objects.create(name="Mr Cleese")
        cls.teacher2 = Teacher.objects.create(name="Mr Idle")
        cls.teacher3 = Teacher.objects.create(name="Mr Chapman")
        cls.teacher1.qualifications.add(cls.qual1, cls.qual2, cls.qual3, cls.qual4)
        cls.teacher2.qualifications.add(cls.qual1)
        cls.teacher3.qualifications.add(cls.qual2)

        cls.dept1 = Department.objects.create(name="English")
        cls.dept2 = Department.objects.create(name="Physics")
        cls.dept1.teachers.add(cls.teacher1, cls.teacher2)
        cls.dept2.teachers.add(cls.teacher1, cls.teacher3)

    def test_m2m_then_m2m(self):
        with self.assertNumQueries(3):
            # When we prefetch the teachers, and force the query, we don't want
            # the default manager on teachers to immediately get all the related
            # qualifications, since this will do one query per teacher.
            qs = Department.objects.prefetch_related("teachers")
            depts = "".join(
                "%s department: %s\n"
                % (dept.name, ", ".join(str(t) for t in dept.teachers.all()))
                for dept in qs
            )

            self.assertEqual(
                depts,
                "English department: Mr Cleese (BA, BSci, MA, PhD), Mr Idle (BA)\n"
                "Physics department: Mr Cleese (BA, BSci, MA, PhD), Mr Chapman "
                "(BSci)\n",
            )


class GenericRelationTests(TestCase):
    @classmethod
    def setUpTestData(cls):
        book1 = Book.objects.create(title="Winnie the Pooh")
        book2 = Book.objects.create(title="Do you like green eggs and spam?")
        book3 = Book.objects.create(title="Three Men In A Boat")

        reader1 = Reader.objects.create(name="me")
        reader2 = Reader.objects.create(name="you")
        reader3 = Reader.objects.create(name="someone")

        book1.read_by.add(reader1, reader2)
        book2.read_by.add(reader2)
        book3.read_by.add(reader3)

        cls.book1, cls.book2, cls.book3 = book1, book2, book3
        cls.reader1, cls.reader2, cls.reader3 = reader1, reader2, reader3

    def test_prefetch_GFK(self):
        TaggedItem.objects.create(tag="awesome", content_object=self.book1)
        TaggedItem.objects.create(tag="great", content_object=self.reader1)
        TaggedItem.objects.create(tag="outstanding", content_object=self.book2)
        TaggedItem.objects.create(tag="amazing", content_object=self.reader3)

        # 1 for TaggedItem table, 1 for Book table, 1 for Reader table
        with self.assertNumQueries(3):
            qs = TaggedItem.objects.prefetch_related("content_object")
            list(qs)

    def test_prefetch_GFK_nonint_pk(self):
        Comment.objects.create(comment="awesome", content_object=self.book1)

        # 1 for Comment table, 1 for Book table
        with self.assertNumQueries(2):
            qs = Comment.objects.prefetch_related("content_object")
            [c.content_object for c in qs]

    def test_prefetch_GFK_uuid_pk(self):
        article = Article.objects.create(name="Django")
        Comment.objects.create(comment="awesome", content_object_uuid=article)
        qs = Comment.objects.prefetch_related("content_object_uuid")
        self.assertEqual([c.content_object_uuid for c in qs], [article])

    def test_prefetch_GFK_fk_pk(self):
        book = Book.objects.create(title="Poems")
        book_with_year = BookWithYear.objects.create(book=book, published_year=2019)
        Comment.objects.create(comment="awesome", content_object=book_with_year)
        qs = Comment.objects.prefetch_related("content_object")
        self.assertEqual([c.content_object for c in qs], [book_with_year])

    def test_traverse_GFK(self):
        """
        A 'content_object' can be traversed with prefetch_related() and
        get to related objects on the other side (assuming it is suitably
        filtered)
        """
        TaggedItem.objects.create(tag="awesome", content_object=self.book1)
        TaggedItem.objects.create(tag="awesome", content_object=self.book2)
        TaggedItem.objects.create(tag="awesome", content_object=self.book3)
        TaggedItem.objects.create(tag="awesome", content_object=self.reader1)
        TaggedItem.objects.create(tag="awesome", content_object=self.reader2)

        ct = ContentType.objects.get_for_model(Book)

        # We get 3 queries - 1 for main query, 1 for content_objects since they
        # all use the same table, and 1 for the 'read_by' relation.
        with self.assertNumQueries(3):
            # If we limit to books, we know that they will have 'read_by'
            # attributes, so the following makes sense:
            qs = TaggedItem.objects.filter(
                content_type=ct, tag="awesome"
            ).prefetch_related("content_object__read_by")
            readers_of_awesome_books = {
                r.name for tag in qs for r in tag.content_object.read_by.all()
            }
            self.assertEqual(readers_of_awesome_books, {"me", "you", "someone"})

    def test_nullable_GFK(self):
        TaggedItem.objects.create(
            tag="awesome", content_object=self.book1, created_by=self.reader1
        )
        TaggedItem.objects.create(tag="great", content_object=self.book2)
        TaggedItem.objects.create(tag="rubbish", content_object=self.book3)

        with self.assertNumQueries(2):
            result = [
                t.created_by for t in TaggedItem.objects.prefetch_related("created_by")
            ]

        self.assertEqual(result, [t.created_by for t in TaggedItem.objects.all()])

    def test_generic_relation(self):
        bookmark = Bookmark.objects.create(url="http://www.djangoproject.com/")
        TaggedItem.objects.create(content_object=bookmark, tag="django")
        TaggedItem.objects.create(content_object=bookmark, tag="python")

        with self.assertNumQueries(2):
            tags = [
                t.tag
                for b in Bookmark.objects.prefetch_related("tags")
                for t in b.tags.all()
            ]
            self.assertEqual(sorted(tags), ["django", "python"])

    def test_charfield_GFK(self):
        b = Bookmark.objects.create(url="http://www.djangoproject.com/")
        TaggedItem.objects.create(content_object=b, tag="django")
        TaggedItem.objects.create(content_object=b, favorite=b, tag="python")

        with self.assertNumQueries(3):
            bookmark = Bookmark.objects.filter(pk=b.pk).prefetch_related(
                "tags", "favorite_tags"
            )[0]
            self.assertEqual(
                sorted(i.tag for i in bookmark.tags.all()), ["django", "python"]
            )
            self.assertEqual([i.tag for i in bookmark.favorite_tags.all()], ["python"])

    def test_custom_queryset(self):
        bookmark = Bookmark.objects.create(url="http://www.djangoproject.com/")
        django_tag = TaggedItem.objects.create(content_object=bookmark, tag="django")
        TaggedItem.objects.create(content_object=bookmark, tag="python")

        with self.assertNumQueries(2):
            bookmark = Bookmark.objects.prefetch_related(
                Prefetch("tags", TaggedItem.objects.filter(tag="django")),
            ).get()

        with self.assertNumQueries(0):
            self.assertEqual(list(bookmark.tags.all()), [django_tag])

        # The custom queryset filters should be applied to the queryset
        # instance returned by the manager.
        self.assertEqual(list(bookmark.tags.all()), list(bookmark.tags.all().all()))

    def test_deleted_GFK(self):
        TaggedItem.objects.create(tag="awesome", content_object=self.book1)
        TaggedItem.objects.create(tag="awesome", content_object=self.book2)
        ct = ContentType.objects.get_for_model(Book)

        book1_pk = self.book1.pk
        self.book1.delete()

        with self.assertNumQueries(2):
            qs = TaggedItem.objects.filter(tag="awesome").prefetch_related(
                "content_object"
            )
            result = [
                (tag.object_id, tag.content_type_id, tag.content_object) for tag in qs
            ]
            self.assertEqual(
                result,
                [
                    (book1_pk, ct.pk, None),
                    (self.book2.pk, ct.pk, self.book2),
                ],
            )


class MultiTableInheritanceTest(TestCase):
    @classmethod
    def setUpTestData(cls):
        cls.book1 = BookWithYear.objects.create(title="Poems", published_year=2010)
        cls.book2 = BookWithYear.objects.create(title="More poems", published_year=2011)
        cls.author1 = AuthorWithAge.objects.create(
            name="Jane", first_book=cls.book1, age=50
        )
        cls.author2 = AuthorWithAge.objects.create(
            name="Tom", first_book=cls.book1, age=49
        )
        cls.author3 = AuthorWithAge.objects.create(
            name="Robert", first_book=cls.book2, age=48
        )
        cls.author_address = AuthorAddress.objects.create(
            author=cls.author1, address="SomeStreet 1"
        )
        cls.book2.aged_authors.add(cls.author2, cls.author3)
        cls.br1 = BookReview.objects.create(book=cls.book1, notes="review book1")
        cls.br2 = BookReview.objects.create(book=cls.book2, notes="review book2")

    def test_foreignkey(self):
        with self.assertNumQueries(2):
            qs = AuthorWithAge.objects.prefetch_related("addresses")
            addresses = [
                [str(address) for address in obj.addresses.all()] for obj in qs
            ]
        self.assertEqual(addresses, [[str(self.author_address)], [], []])

    def test_foreignkey_to_inherited(self):
        with self.assertNumQueries(2):
            qs = BookReview.objects.prefetch_related("book")
            titles = [obj.book.title for obj in qs]
        self.assertCountEqual(titles, ["Poems", "More poems"])

    def test_m2m_to_inheriting_model(self):
        qs = AuthorWithAge.objects.prefetch_related("books_with_year")
        with self.assertNumQueries(2):
            lst = [
                [str(book) for book in author.books_with_year.all()] for author in qs
            ]
        qs = AuthorWithAge.objects.all()
        lst2 = [[str(book) for book in author.books_with_year.all()] for author in qs]
        self.assertEqual(lst, lst2)

        qs = BookWithYear.objects.prefetch_related("aged_authors")
        with self.assertNumQueries(2):
            lst = [[str(author) for author in book.aged_authors.all()] for book in qs]
        qs = BookWithYear.objects.all()
        lst2 = [[str(author) for author in book.aged_authors.all()] for book in qs]
        self.assertEqual(lst, lst2)

    def test_parent_link_prefetch(self):
        with self.assertNumQueries(2):
            [a.author for a in AuthorWithAge.objects.prefetch_related("author")]

    @override_settings(DEBUG=True)
    def test_child_link_prefetch(self):
        with self.assertNumQueries(2):
            authors = [
                a.authorwithage
                for a in Author.objects.prefetch_related("authorwithage")
            ]

        # Regression for #18090: the prefetching query must include an IN clause.
        # Note that on Oracle the table name is upper case in the generated SQL,
        # thus the .lower() call.
        self.assertIn("authorwithage", connection.queries[-1]["sql"].lower())
        self.assertIn(" IN ", connection.queries[-1]["sql"])

        self.assertEqual(authors, [a.authorwithage for a in Author.objects.all()])


class ForeignKeyToFieldTest(TestCase):
    @classmethod
    def setUpTestData(cls):
        cls.book = Book.objects.create(title="Poems")
        cls.author1 = Author.objects.create(name="Jane", first_book=cls.book)
        cls.author2 = Author.objects.create(name="Tom", first_book=cls.book)
        cls.author3 = Author.objects.create(name="Robert", first_book=cls.book)
        cls.author_address = AuthorAddress.objects.create(
            author=cls.author1, address="SomeStreet 1"
        )
        FavoriteAuthors.objects.create(author=cls.author1, likes_author=cls.author2)
        FavoriteAuthors.objects.create(author=cls.author2, likes_author=cls.author3)
        FavoriteAuthors.objects.create(author=cls.author3, likes_author=cls.author1)

    def test_foreignkey(self):
        with self.assertNumQueries(2):
            qs = Author.objects.prefetch_related("addresses")
            addresses = [
                [str(address) for address in obj.addresses.all()] for obj in qs
            ]
        self.assertEqual(addresses, [[str(self.author_address)], [], []])

    def test_m2m(self):
        with self.assertNumQueries(3):
            qs = Author.objects.prefetch_related("favorite_authors", "favors_me")
            favorites = [
                (
                    [str(i_like) for i_like in author.favorite_authors.all()],
                    [str(likes_me) for likes_me in author.favors_me.all()],
                )
                for author in qs
            ]
            self.assertEqual(
                favorites,
                [
                    ([str(self.author2)], [str(self.author3)]),
                    ([str(self.author3)], [str(self.author1)]),
                    ([str(self.author1)], [str(self.author2)]),
                ],
            )


class LookupOrderingTest(TestCase):
    """
    Test cases that demonstrate that ordering of lookups is important, and
    ensure it is preserved.
    """

    @classmethod
    def setUpTestData(cls):
        person1 = Person.objects.create(name="Joe")
        person2 = Person.objects.create(name="Mary")

        # Set main_room for each house before creating the next one for
        # databases where supports_nullable_unique_constraints is False.
        house1 = House.objects.create(address="123 Main St")
        room1_1 = Room.objects.create(name="Dining room", house=house1)
        Room.objects.create(name="Lounge", house=house1)
        Room.objects.create(name="Kitchen", house=house1)
        house1.main_room = room1_1
        house1.save()
        person1.houses.add(house1)

        house2 = House.objects.create(address="45 Side St")
        room2_1 = Room.objects.create(name="Dining room", house=house2)
        Room.objects.create(name="Lounge", house=house2)
        house2.main_room = room2_1
        house2.save()
        person1.houses.add(house2)

        house3 = House.objects.create(address="6 Downing St")
        room3_1 = Room.objects.create(name="Dining room", house=house3)
        Room.objects.create(name="Lounge", house=house3)
        Room.objects.create(name="Kitchen", house=house3)
        house3.main_room = room3_1
        house3.save()
        person2.houses.add(house3)

        house4 = House.objects.create(address="7 Regents St")
        room4_1 = Room.objects.create(name="Dining room", house=house4)
        Room.objects.create(name="Lounge", house=house4)
        house4.main_room = room4_1
        house4.save()
        person2.houses.add(house4)

    def test_order(self):
        with self.assertNumQueries(4):
            # The following two queries must be done in the same order as written,
            # otherwise 'primary_house' will cause non-prefetched lookups
            qs = Person.objects.prefetch_related(
                "houses__rooms", "primary_house__occupants"
            )
            [list(p.primary_house.occupants.all()) for p in qs]


class NullableTest(TestCase):
    @classmethod
    def setUpTestData(cls):
        boss = Employee.objects.create(name="Peter")
        Employee.objects.create(name="Joe", boss=boss)
        Employee.objects.create(name="Angela", boss=boss)

    def test_traverse_nullable(self):
        # Because we use select_related() for 'boss', it doesn't need to be
        # prefetched, but we can still traverse it although it contains some nulls
        with self.assertNumQueries(2):
            qs = Employee.objects.select_related("boss").prefetch_related("boss__serfs")
            co_serfs = [
                list(e.boss.serfs.all()) if e.boss is not None else [] for e in qs
            ]

        qs2 = Employee.objects.select_related("boss")
        co_serfs2 = [
            list(e.boss.serfs.all()) if e.boss is not None else [] for e in qs2
        ]

        self.assertEqual(co_serfs, co_serfs2)

    def test_prefetch_nullable(self):
        # One for main employee, one for boss, one for serfs
        with self.assertNumQueries(3):
            qs = Employee.objects.prefetch_related("boss__serfs")
            co_serfs = [
                list(e.boss.serfs.all()) if e.boss is not None else [] for e in qs
            ]

        qs2 = Employee.objects.all()
        co_serfs2 = [
            list(e.boss.serfs.all()) if e.boss is not None else [] for e in qs2
        ]

        self.assertEqual(co_serfs, co_serfs2)

    def test_in_bulk(self):
        """
        In-bulk does correctly prefetch objects by not using .iterator()
        directly.
        """
        boss1 = Employee.objects.create(name="Peter")
        boss2 = Employee.objects.create(name="Jack")
        with self.assertNumQueries(2):
            # Prefetch is done and it does not cause any errors.
            bulk = Employee.objects.prefetch_related("serfs").in_bulk(
                [boss1.pk, boss2.pk]
            )
            for b in bulk.values():
                list(b.serfs.all())


class MultiDbTests(TestCase):
    databases = {"default", "other"}

    def test_using_is_honored_m2m(self):
        B = Book.objects.using("other")
        A = Author.objects.using("other")
        book1 = B.create(title="Poems")
        book2 = B.create(title="Jane Eyre")
        book3 = B.create(title="Wuthering Heights")
        book4 = B.create(title="Sense and Sensibility")

        author1 = A.create(name="Charlotte", first_book=book1)
        author2 = A.create(name="Anne", first_book=book1)
        author3 = A.create(name="Emily", first_book=book1)
        author4 = A.create(name="Jane", first_book=book4)

        book1.authors.add(author1, author2, author3)
        book2.authors.add(author1)
        book3.authors.add(author3)
        book4.authors.add(author4)

        # Forward
        qs1 = B.prefetch_related("authors")
        with self.assertNumQueries(2, using="other"):
            books = "".join(
                "%s (%s)\n"
                % (book.title, ", ".join(a.name for a in book.authors.all()))
                for book in qs1
            )
        self.assertEqual(
            books,
            "Poems (Charlotte, Anne, Emily)\n"
            "Jane Eyre (Charlotte)\n"
            "Wuthering Heights (Emily)\n"
            "Sense and Sensibility (Jane)\n",
        )

        # Reverse
        qs2 = A.prefetch_related("books")
        with self.assertNumQueries(2, using="other"):
            authors = "".join(
                "%s: %s\n"
                % (author.name, ", ".join(b.title for b in author.books.all()))
                for author in qs2
            )
        self.assertEqual(
            authors,
            "Charlotte: Poems, Jane Eyre\n"
            "Anne: Poems\n"
            "Emily: Poems, Wuthering Heights\n"
            "Jane: Sense and Sensibility\n",
        )

    def test_using_is_honored_fkey(self):
        B = Book.objects.using("other")
        A = Author.objects.using("other")
        book1 = B.create(title="Poems")
        book2 = B.create(title="Sense and Sensibility")

        A.create(name="Charlotte Bronte", first_book=book1)
        A.create(name="Jane Austen", first_book=book2)

        # Forward
        with self.assertNumQueries(2, using="other"):
            books = ", ".join(
                a.first_book.title for a in A.prefetch_related("first_book")
            )
        self.assertEqual("Poems, Sense and Sensibility", books)

        # Reverse
        with self.assertNumQueries(2, using="other"):
            books = "".join(
                "%s (%s)\n"
                % (b.title, ", ".join(a.name for a in b.first_time_authors.all()))
                for b in B.prefetch_related("first_time_authors")
            )
        self.assertEqual(
            books,
            "Poems (Charlotte Bronte)\nSense and Sensibility (Jane Austen)\n",
        )

    def test_using_is_honored_inheritance(self):
        B = BookWithYear.objects.using("other")
        A = AuthorWithAge.objects.using("other")
        book1 = B.create(title="Poems", published_year=2010)
        B.create(title="More poems", published_year=2011)
        A.create(name="Jane", first_book=book1, age=50)
        A.create(name="Tom", first_book=book1, age=49)

        # parent link
        with self.assertNumQueries(2, using="other"):
            authors = ", ".join(a.author.name for a in A.prefetch_related("author"))

        self.assertEqual(authors, "Jane, Tom")

        # child link
        with self.assertNumQueries(2, using="other"):
            ages = ", ".join(
                str(a.authorwithage.age) for a in A.prefetch_related("authorwithage")
            )

        self.assertEqual(ages, "50, 49")

    def test_using_is_honored_custom_qs(self):
        B = Book.objects.using("other")
        A = Author.objects.using("other")
        book1 = B.create(title="Poems")
        book2 = B.create(title="Sense and Sensibility")

        A.create(name="Charlotte Bronte", first_book=book1)
        A.create(name="Jane Austen", first_book=book2)

        # Implicit hinting
        with self.assertNumQueries(2, using="other"):
            prefetch = Prefetch("first_time_authors", queryset=Author.objects.all())
            books = "".join(
                "%s (%s)\n"
                % (b.title, ", ".join(a.name for a in b.first_time_authors.all()))
                for b in B.prefetch_related(prefetch)
            )
        self.assertEqual(
            books,
            "Poems (Charlotte Bronte)\nSense and Sensibility (Jane Austen)\n",
        )
        # Explicit using on the same db.
        with self.assertNumQueries(2, using="other"):
            prefetch = Prefetch(
                "first_time_authors", queryset=Author.objects.using("other")
            )
            books = "".join(
                "%s (%s)\n"
                % (b.title, ", ".join(a.name for a in b.first_time_authors.all()))
                for b in B.prefetch_related(prefetch)
            )
        self.assertEqual(
            books,
            "Poems (Charlotte Bronte)\nSense and Sensibility (Jane Austen)\n",
        )

        # Explicit using on a different db.
        with (
            self.assertNumQueries(1, using="default"),
            self.assertNumQueries(1, using="other"),
        ):
            prefetch = Prefetch(
                "first_time_authors", queryset=Author.objects.using("default")
            )
            books = "".join(
                "%s (%s)\n"
                % (b.title, ", ".join(a.name for a in b.first_time_authors.all()))
                for b in B.prefetch_related(prefetch)
            )
        self.assertEqual(books, "Poems ()\n" "Sense and Sensibility ()\n")


class Ticket19607Tests(TestCase):
    @classmethod
    def setUpTestData(cls):
        LessonEntry.objects.bulk_create(
            LessonEntry(id=id_, name1=name1, name2=name2)
            for id_, name1, name2 in [
                (1, "einfach", "simple"),
                (2, "schwierig", "difficult"),
            ]
        )
        WordEntry.objects.bulk_create(
            WordEntry(id=id_, lesson_entry_id=lesson_entry_id, name=name)
            for id_, lesson_entry_id, name in [
                (1, 1, "einfach"),
                (2, 1, "simple"),
                (3, 2, "schwierig"),
                (4, 2, "difficult"),
            ]
        )

    def test_bug(self):
        list(
            WordEntry.objects.prefetch_related(
                "lesson_entry", "lesson_entry__wordentry_set"
            )
        )


class Ticket21410Tests(TestCase):
    @classmethod
    def setUpTestData(cls):
        book1 = Book.objects.create(title="Poems")
        book2 = Book.objects.create(title="Jane Eyre")
        book3 = Book.objects.create(title="Wuthering Heights")
        book4 = Book.objects.create(title="Sense and Sensibility")

        author1 = Author2.objects.create(name="Charlotte", first_book=book1)
        author2 = Author2.objects.create(name="Anne", first_book=book1)
        author3 = Author2.objects.create(name="Emily", first_book=book1)
        author4 = Author2.objects.create(name="Jane", first_book=book4)

        author1.favorite_books.add(book1, book2, book3)
        author2.favorite_books.add(book1)
        author3.favorite_books.add(book2)
        author4.favorite_books.add(book3)

    def test_bug(self):
        list(Author2.objects.prefetch_related("first_book", "favorite_books"))


class Ticket21760Tests(TestCase):
    @classmethod
    def setUpTestData(cls):
        cls.rooms = []
        for _ in range(3):
            house = House.objects.create()
            for _ in range(3):
                cls.rooms.append(Room.objects.create(house=house))
            # Set main_room for each house before creating the next one for
            # databases where supports_nullable_unique_constraints is False.
            house.main_room = cls.rooms[-3]
            house.save()

    def test_bug(self):
        prefetcher = get_prefetcher(self.rooms[0], "house", "house")[0]
        queryset = prefetcher.get_prefetch_querysets(list(Room.objects.all()))[0]
        self.assertNotIn(" JOIN ", str(queryset.query))


class DirectPrefetchedObjectCacheReuseTests(TestCase):
    """
    prefetch_related() reuses objects fetched in _prefetched_objects_cache.

    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
    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"),
        ]
        cls.bookwithyear1 = BookWithYear.objects.create(
            title="Poems", published_year=2010
        )
        cls.bookreview1 = BookReview.objects.create(book=cls.bookwithyear1)

    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"]).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.assertSequenceEqual(
                book1.first_time_authors.all(), [self.author11, self.author12]
            )
            self.assertSequenceEqual(book2.first_time_authors.all(), [self.author21])

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

        self.assertEqual(
            list(book1.first_time_authors.all()),
            list(book1.first_time_authors.all().all()),
        )
        self.assertEqual(
            list(book2.first_time_authors.all()),
            list(book2.first_time_authors.all().all()),
        )
        self.assertEqual(
            list(book1.first_time_authors.all()[0].addresses.all()),
            list(book1.first_time_authors.all()[0].addresses.all().all()),
        )
        self.assertEqual(
            list(book1.first_time_authors.all()[1].addresses.all()),
            list(book1.first_time_authors.all()[1].addresses.all().all()),
        )
        self.assertEqual(
            list(book2.first_time_authors.all()[0].addresses.all()),
            list(book2.first_time_authors.all()[0].addresses.all().all()),
        )

    def test_detect_is_fetched_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.assertEqual(book1.first_authors, [self.author11, self.author12])
            self.assertEqual(book2.first_authors, [self.author21])

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

    def test_prefetch_reverse_foreign_key(self):
        with self.assertNumQueries(2):
            (bookwithyear1,) = BookWithYear.objects.prefetch_related("bookreview_set")
        with self.assertNumQueries(0):
            self.assertCountEqual(
                bookwithyear1.bookreview_set.all(), [self.bookreview1]
            )
        with self.assertNumQueries(0):
            prefetch_related_objects([bookwithyear1], "bookreview_set")

    def test_add_clears_prefetched_objects(self):
        bookwithyear = BookWithYear.objects.get(pk=self.bookwithyear1.pk)
        prefetch_related_objects([bookwithyear], "bookreview_set")
        self.assertCountEqual(bookwithyear.bookreview_set.all(), [self.bookreview1])
        new_review = BookReview.objects.create()
        bookwithyear.bookreview_set.add(new_review)
        self.assertCountEqual(
            bookwithyear.bookreview_set.all(), [self.bookreview1, new_review]
        )

    def test_remove_clears_prefetched_objects(self):
        bookwithyear = BookWithYear.objects.get(pk=self.bookwithyear1.pk)
        prefetch_related_objects([bookwithyear], "bookreview_set")
        self.assertCountEqual(bookwithyear.bookreview_set.all(), [self.bookreview1])
        bookwithyear.bookreview_set.remove(self.bookreview1)
        self.assertCountEqual(bookwithyear.bookreview_set.all(), [])


class ReadPrefetchedObjectsCacheTests(TestCase):
    @classmethod
    def setUpTestData(cls):
        cls.book1 = Book.objects.create(title="Les confessions Volume I")
        cls.book2 = Book.objects.create(title="Candide")
        cls.author1 = AuthorWithAge.objects.create(
            name="Rousseau", first_book=cls.book1, age=70
        )
        cls.author2 = AuthorWithAge.objects.create(
            name="Voltaire", first_book=cls.book2, age=65
        )
        cls.book1.authors.add(cls.author1)
        cls.book2.authors.add(cls.author2)
        FavoriteAuthors.objects.create(author=cls.author1, likes_author=cls.author2)

    def test_retrieves_results_from_prefetched_objects_cache(self):
        """
        When intermediary results are prefetched without a destination
        attribute, they are saved in the RelatedManager's cache
        (_prefetched_objects_cache). prefetch_related() uses this cache
        (#27554).
        """
        authors = AuthorWithAge.objects.prefetch_related(
            Prefetch(
                "author",
                queryset=Author.objects.prefetch_related(
                    # Results are saved in the RelatedManager's cache
                    # (_prefetched_objects_cache) and do not replace the
                    # RelatedManager on Author instances (favorite_authors)
                    Prefetch("favorite_authors__first_book"),
                ),
            ),
        )
        with self.assertNumQueries(4):
            # AuthorWithAge -> Author -> FavoriteAuthors, Book
            self.assertSequenceEqual(authors, [self.author1, self.author2])


class NestedPrefetchTests(TestCase):
    @classmethod
    def setUpTestData(cls):
        house = House.objects.create(name="Big house", address="123 Main St")
        cls.room = Room.objects.create(name="Kitchen", house=house)

    def test_nested_prefetch_is_not_overwritten_by_related_object(self):
        """
        The prefetched relationship is used rather than populating the reverse
        relationship from the parent, when prefetching a set of child objects
        related to a set of parent objects and the child queryset itself
        specifies a prefetch back to the parent.
        """
        queryset = House.objects.only("name").prefetch_related(
            Prefetch(
                "rooms",
                queryset=Room.objects.prefetch_related(
                    Prefetch("house", queryset=House.objects.only("address")),
                ),
            ),
        )
        with self.assertNumQueries(3):
            house = queryset.first()

        self.assertIs(Room.house.is_cached(self.room), True)
        with self.assertNumQueries(0):
            house.rooms.first().house.address


class PrefetchLimitTests(TestDataMixin, TestCase):
    @skipUnlessDBFeature("supports_over_clause")
    def test_m2m_forward(self):
        authors = Author.objects.all()  # Meta.ordering
        with self.assertNumQueries(3):
            books = list(
                Book.objects.prefetch_related(
                    Prefetch("authors", authors),
                    Prefetch("authors", authors[1:], to_attr="authors_sliced"),
                )
            )
        for book in books:
            with self.subTest(book=book):
                self.assertEqual(book.authors_sliced, list(book.authors.all())[1:])

    @skipUnlessDBFeature("supports_over_clause")
    def test_m2m_reverse(self):
        books = Book.objects.order_by("title")
        with self.assertNumQueries(3):
            authors = list(
                Author.objects.prefetch_related(
                    Prefetch("books", books),
                    Prefetch("books", books[1:2], to_attr="books_sliced"),
                )
            )
        for author in authors:
            with self.subTest(author=author):
                self.assertEqual(author.books_sliced, list(author.books.all())[1:2])

    @skipUnlessDBFeature("supports_over_clause")
    def test_foreignkey_reverse(self):
        authors = Author.objects.order_by("-name")
        with self.assertNumQueries(3):
            books = list(
                Book.objects.prefetch_related(
                    Prefetch(
                        "first_time_authors",
                        authors,
                    ),
                    Prefetch(
                        "first_time_authors",
                        authors[1:],
                        to_attr="first_time_authors_sliced",
                    ),
                )
            )
        for book in books:
            with self.subTest(book=book):
                self.assertEqual(
                    book.first_time_authors_sliced,
                    list(book.first_time_authors.all())[1:],
                )

    @skipUnlessDBFeature("supports_over_clause")
    def test_reverse_ordering(self):
        authors = Author.objects.reverse()  # Reverse Meta.ordering
        with self.assertNumQueries(3):
            books = list(
                Book.objects.prefetch_related(
                    Prefetch("authors", authors),
                    Prefetch("authors", authors[1:], to_attr="authors_sliced"),
                )
            )
        for book in books:
            with self.subTest(book=book):
                self.assertEqual(book.authors_sliced, list(book.authors.all())[1:])

    @skipIfDBFeature("supports_over_clause")
    def test_window_not_supported(self):
        authors = Author.objects.all()
        msg = (
            "Prefetching from a limited queryset is only supported on backends that "
            "support window functions."
        )
        with self.assertRaisesMessage(NotSupportedError, msg):
            list(Book.objects.prefetch_related(Prefetch("authors", authors[1:])))

    @skipUnlessDBFeature("supports_over_clause")
    def test_empty_order(self):
        authors = Author.objects.order_by()
        with self.assertNumQueries(3):
            books = list(
                Book.objects.prefetch_related(
                    Prefetch("authors", authors),
                    Prefetch("authors", authors[:1], to_attr="authors_sliced"),
                )
            )
        for book in books:
            with self.subTest(book=book):
                self.assertEqual(len(book.authors_sliced), 1)
                self.assertIn(book.authors_sliced[0], list(book.authors.all()))


class DeprecationTests(TestCase):
    def test_get_current_queryset_warning(self):
        msg = (
            "Prefetch.get_current_queryset() is deprecated. Use "
            "get_current_querysets() instead."
        )
        authors = Author.objects.all()
        with self.assertWarnsMessage(RemovedInDjango60Warning, msg) as ctx:
            self.assertEqual(
                Prefetch("authors", authors).get_current_queryset(1),
                authors,
            )
        self.assertEqual(ctx.filename, __file__)
        with self.assertWarnsMessage(RemovedInDjango60Warning, msg) as ctx:
            self.assertIsNone(Prefetch("authors").get_current_queryset(1))
        self.assertEqual(ctx.filename, __file__)

    @ignore_warnings(category=RemovedInDjango60Warning)
    def test_prefetch_one_level_fallback(self):
        class NoGetPrefetchQuerySetsDescriptor(ForwardManyToOneDescriptor):
            def get_prefetch_queryset(self, instances, queryset=None):
                if queryset is None:
                    return super().get_prefetch_querysets(instances)
                return super().get_prefetch_querysets(instances, [queryset])

            def __getattribute__(self, name):
                if name == "get_prefetch_querysets":
                    raise AttributeError
                return super().__getattribute__(name)

        house = House.objects.create()
        room = Room.objects.create(house=house)
        house.main_room = room
        house.save()

        # prefetch_one_level() fallbacks to get_prefetch_queryset().
        prefetcher = NoGetPrefetchQuerySetsDescriptor(Room._meta.get_field("house"))
        obj_list, additional_lookups = prefetch_one_level(
            [room],
            prefetcher,
            Prefetch("house", House.objects.all()),
            0,
        )
        self.assertEqual(obj_list, [house])
        self.assertEqual(additional_lookups, [])

        obj_list, additional_lookups = prefetch_one_level(
            [room],
            prefetcher,
            Prefetch("house"),
            0,
        )
        self.assertEqual(obj_list, [house])
        self.assertEqual(additional_lookups, [])