select_related在prefetch_related之后 [英] select_related after prefetch_related

问题描述

我的模型如下：

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：

And I am trying to run a query like this:

[[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应该能够执行两个查询：（ Page JOIN Baz ）和图书。最后的联接应在Python中进行。相反，我看到ORM执行三个查询。我相信我必须在某种程度上使用 select_related ，但这还是行不通的（在 prefetch_related ）：

Which I think the ORM should be able to perform as two queries: (Page JOIN Baz) and Book. The final join should occur in Python. Instead I see the ORM executing three queries. I believe that I have to some how use select_related, but this does not work either (nor does moving select_related before prefetch_related):

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,)

这与此处的注释有关：针对多个级别的prefetch_related

This is related to the comment here: prefetch_related for multiple Levels

推荐答案

您可以尝试使用 Prefetch 对象

You can try to use 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))

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