为什么django的prefetch_related（）仅适用于all（）而不是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）
 
类照片（models.Model）：
 album = models.ForeignKey（'PhotoAlbum'）
 format = models.IntegerField（）
  

现在，如果我想看一下照片的一部分在专辑的一个子集有效率。我做这样的事情：

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

这只有两个查询，这是我期望的（一个获得相册，然后一个像SELECT * IN照片，其中photoalbum_id IN（）。

但是，如果我这样做：

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

然后它执行大量查询， WHERE format = 1 ！我做错了什么，或者django不够聪明，意识到它已经获取了所有的照片，并可以在python中过滤它们？我发誓我在文档中读到应该做的那个...

解决方案

在Django 1.6及更早版本中，无法避免额外的查询。 prefetch_related 调用有效地缓存了查询器中每个专辑的 a.photoset.all（）的结果。但是， a.photoset.filter（format = 1）是一个不同的查询器，因此您将为每个专辑生成一个额外的查询。

这在 prefetch_related 文档。 filter（format = 1）相当于 filter（spicy = True）。

请注意，您可以通过过滤python中的照片来减少数量或查询：

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

在Django 1.7中， a href =https://docs.djangoproject.com/en/1.7/ref/models/queries/#django.db.models.Prefetch =noreferrer> Prefetch（） 对象，允许您控制 prefetch_related 的行为。

  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 someAlbums： 
 somePhotos = a.some_photos 
  

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

suppose I have this model:

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()

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()

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 ().

Everything is great.

But if I do this:

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

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...

解决方案

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.photoset.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.

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