为什么 django 的 prefetch_related() 只适用于 all() 而不适用于 filter()? [英] Why does django's prefetch_related() only work with all() and not filter()?

问题描述

假设我有这个模型:

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相册子集中的照片子集.我这样做是这样的:

Now if I want to look at a subset of photos in a subset of albums efficiently. I do it something like this:

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

这只做了两个查询，这正是我所期望的(一个是获取相册，然后一个像`SELECT * IN photos WHERE photoalbum_id IN ().

This does only two queries, which is what I expect (one to get the albums, and then one like `SELECT * IN photos WHERE photoalbum_id IN ().

一切都很棒.

但是如果我这样做:

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

然后它使用 WHERE format = 1 进行大量查询！我做错了什么还是django不够聪明以意识到它已经获取了所有照片并且可以在python中过滤它们?我发誓我在文档中的某处读到它应该这样做......

Then it does a ton of queries with WHERE format = 1! Am I doing something wrong or is django not smart enough to realise it has already fetched all the photos and can filter them in python? I swear I read somewhere in the documentation that it is supposed to do that...

推荐答案

在 Django 1.6 及更早版本中，无法避免额外的查询.prefetch_related 调用有效地为查询集中的每个相册缓存了 a.photoset.all() 的结果.但是，a.photoset.filter(format=1) 是一个不同的查询集，因此您将为每个相册生成一个额外的查询.

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.

这在 prefetch_related<中有解释/code> 文档.filter(format=1) 等价于 filter(spicy=True).

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

请注意，您可以通过在 python 中过滤照片来减少数量或查询:

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]

在 Django 1.7 中，有一个 Prefetch() 对象，允许您控制 prefetch_related 的行为.

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

有关如何使用 Prefetch 对象的更多示例，请参阅 prefetch_related 文档.

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

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