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来执行:(Page JOIN Baz
) and Book
。最后的连接应该在 Python 中进行。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(移动也不起作用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,)
这与这里的评论相关:多个级别的预取相关 https://stackoverflow.com/questions/27116770/prefetch-related-for-multiple-levels#comment42737662_27117246