django/tests/prefetch_related/test_prefetch_related_objects.py

from django.db.models import Prefetch, prefetch_related_objects
from django.test import TestCase

from .models import Author, Book, Reader


class PrefetchRelatedObjectsTests(TestCase):
    """
    Since prefetch_related_objects() is just the inner part of
    prefetch_related(), only do basic tests to ensure its API hasn't changed.
    """

    @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)

    def test_unknown(self):
        book1 = Book.objects.get(id=self.book1.id)
        with self.assertRaises(AttributeError):
            prefetch_related_objects([book1], "unknown_attribute")

    def test_m2m_forward(self):
        book1 = Book.objects.get(id=self.book1.id)
        with self.assertNumQueries(1):
            prefetch_related_objects([book1], "authors")

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

    def test_m2m_reverse(self):
        author1 = Author.objects.get(id=self.author1.id)
        with self.assertNumQueries(1):
            prefetch_related_objects([author1], "books")

        with self.assertNumQueries(0):
            self.assertCountEqual(author1.books.all(), [self.book1, self.book2])

    def test_foreignkey_forward(self):
        authors = list(Author.objects.all())
        with self.assertNumQueries(1):
            prefetch_related_objects(authors, "first_book")

        with self.assertNumQueries(0):
            [author.first_book for author in authors]

    def test_foreignkey_reverse(self):
        books = list(Book.objects.all())
        with self.assertNumQueries(1):
            prefetch_related_objects(books, "first_time_authors")

        with self.assertNumQueries(0):
            [list(book.first_time_authors.all()) for book in books]

    def test_m2m_then_m2m(self):
        """A m2m can be followed through another m2m."""
        authors = list(Author.objects.all())
        with self.assertNumQueries(2):
            prefetch_related_objects(authors, "books__read_by")

        with self.assertNumQueries(0):
            self.assertEqual(
                [
                    [[str(r) for r in b.read_by.all()] for b in a.books.all()]
                    for a in authors
                ],
                [
                    [["Amy"], ["Belinda"]],  # Charlotte - Poems, Jane Eyre
                    [["Amy"]],  # Anne - Poems
                    [["Amy"], []],  # Emily - Poems, Wuthering Heights
                    [["Amy", "Belinda"]],  # Jane - Sense and Sense
                ],
            )

    def test_prefetch_object(self):
        book1 = Book.objects.get(id=self.book1.id)
        with self.assertNumQueries(1):
            prefetch_related_objects([book1], Prefetch("authors"))

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

    def test_prefetch_object_twice(self):
        book1 = Book.objects.get(id=self.book1.id)
        book2 = Book.objects.get(id=self.book2.id)
        with self.assertNumQueries(1):
            prefetch_related_objects([book1], Prefetch("authors"))
        with self.assertNumQueries(1):
            prefetch_related_objects([book1, book2], Prefetch("authors"))
        with self.assertNumQueries(0):
            self.assertCountEqual(book2.authors.all(), [self.author1])

    def test_prefetch_object_to_attr(self):
        book1 = Book.objects.get(id=self.book1.id)
        with self.assertNumQueries(1):
            prefetch_related_objects(
                [book1], Prefetch("authors", to_attr="the_authors")
            )

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

    def test_prefetch_object_to_attr_twice(self):
        book1 = Book.objects.get(id=self.book1.id)
        book2 = Book.objects.get(id=self.book2.id)
        with self.assertNumQueries(1):
            prefetch_related_objects(
                [book1],
                Prefetch("authors", to_attr="the_authors"),
            )
        with self.assertNumQueries(1):
            prefetch_related_objects(
                [book1, book2],
                Prefetch("authors", to_attr="the_authors"),
            )
        with self.assertNumQueries(0):
            self.assertCountEqual(book2.the_authors, [self.author1])

    def test_prefetch_queryset(self):
        book1 = Book.objects.get(id=self.book1.id)
        with self.assertNumQueries(1):
            prefetch_related_objects(
                [book1],
                Prefetch(
                    "authors",
                    queryset=Author.objects.filter(
                        id__in=[self.author1.id, self.author2.id]
                    ),
                ),
            )

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