prefetch_related 之后的 select_related

Question

我的模型看起来像：

class Book(models.Model):
    publisher = models.ForeignKey(Publisher) # This is not important

class Baz(models.Model):
    a = models.CharField(max_length=100)

class Page(models.Model):
    book = models.ForeignKey(Book)
    baz = models.ForeignKey(Baz)

我正在尝试运行这样的查询：

[[x.baz.a for x in y.page_set.all()]
for y in Book.objects.all().prefetch_related('page_set', 'page_set__baz')]

我认为 ORM 应该能够作为两个查询执行：(Page JOIN Baz) 和 Book。最后的连接应该发生在 Python 中。相反，我看到 ORM 执行三个查询。我相信我必须知道如何使用select_related，但这也不起作用（在prefetch_related 之前移动select_related 也不起作用）：

Book.objects.all().prefetch_related('page_set', 'page_set__baz').select_related('page_set__baz')

查询（uploads 是应用程序）：

QUERY = u'SELECT "uploads_book"."id", "uploads_book"."publisher_id" FROM "uploads_book"' - PARAMS = (); args=()
QUERY = u'SELECT "uploads_page"."id", "uploads_page"."book_id", "uploads_page"."baz_id" FROM "uploads_page" WHERE "uploads_page"."book_id" IN (%s, %s)' - PARAMS = (1, 2); args=(1, 2)
QUERY = u'SELECT "uploads_baz"."id", "uploads_baz"."a" FROM "uploads_baz" WHERE "uploads_baz"."id" IN (%s)' - PARAMS = (1,); args=(1,)

这与这里的评论有关：prefetch_related for multiple Levels

Answer 1

你可以尝试使用Prefetch object

from django.db.models import Prefetch

page_qs = Page.objects.select_related('baz')
books = Book.objects.prefetch_related(Prefetch('page_set', queryset=page_qs))