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我的模型看起来像:select_related prefetch_related后
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)
,我试图运行这样的查询:
[[x.baz.a for x in y.page_set.all()]
for y in Book.objects.all().prefetch_related('page_set', 'page_set__baz')]
其中我认为ORM应该能够为两个查询执行: (Page JOIN Baz
)和Book
。最终的连接应该在Python中进行。相反,我看到ORM执行三个查询。我相信,我有一些如何使用select_related
,但这并不工作,要么(也不prefetch_related
前移动select_related
):
Book.objects.all().prefetch_related('page_set', 'page_set__baz').select_related('page_set__baz')
的查询(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,)
这是关系到在这里评论:prefetch_related for multiple Levels