为什么 django 的预取相关()只能用于 all()而不能用于 filter() ？

假设我有这样一个模型:

class PhotoAlbum(models.Model):
title = models.CharField(max_length=128)
author = models.CharField(max_length=128)


class Photo(models.Model):
album = models.ForeignKey('PhotoAlbum')
format = models.IntegerField()

现在，如果我想有效地查看一部分相册中的一部分照片。我是这样做的:

someAlbums = PhotoAlbum.objects.filter(author="Davey Jones").prefetch_related("photo_set")
for a in someAlbums:
somePhotos = a.photo_set.all()

这只执行两个查询，这正是我所期望的(一个查询获取相册，然后一个像‘ SELECT * IN photos WHERE photolab _ id IN ()。

一切都很好。

但如果我这么做:

someAlbums = PhotoAlbum.objects.filter(author="Davey Jones").prefetch_related("photo_set")
for a in someAlbums:
somePhotos = a.photo_set.filter(format=1)

然后，它做了大量的查询与 WHERE format = 1！是我做错了什么，还是 django 不够聪明，没有意识到它已经获取了所有的照片，并且可以在 python 中过滤它们？我发誓我在文件里看到过它应该这么做。

80895

小开

最佳答案

In Django 1.6 and earlier, it is not possible to avoid the extra queries. The prefetch_related call effectively caches the results of a.photoset.all() for every album in the queryset. However, a.photoset.filter(format=1) is a different queryset, so you will generate an extra query for every album.

This is explained in the prefetch_related docs. The filter(format=1) is equivalent to filter(spicy=True).

Note that you could reduce the number or queries by filtering the photos in python instead:

someAlbums = PhotoAlbum.objects.filter(author="Davey Jones").prefetch_related("photo_set")
for a in someAlbums:
somePhotos = [p for p in a.photo_set.all() if p.format == 1]

In Django 1.7, there is a Prefetch() object that allows you to control the behaviour of prefetch_related.

from django.db.models import Prefetch


someAlbums = PhotoAlbum.objects.filter(author="Davey Jones").prefetch_related(
Prefetch(
"photo_set",
queryset=Photo.objects.filter(format=1),
to_attr="some_photos"
)
)
for a in someAlbums:
somePhotos = a.some_photos

For more examples of how to use the Prefetch object, see the prefetch_related docs.