prefetch_related为多个级别 [英] prefetch_related for multiple Levels

问题描述

如果我的模型看起来像：

If my Models look like:

class Publisher(models.Model):
    pass

class Book(models.Model):
    publisher = models.ForeignKey(Publisher)

class Page(models.Model):
    book = models.ForeignKey(Book)

我想获取发布者我做 Publisher.object.all（）。
如果然后想要确保预取我可以做：

and I would like to get the queryset for Publisher I do Publisher.object.all(). If then want to make sure to prefetch I can do:

Publisher.objects.all().prefetch_related('book_set')`

我的问题是：

1. 有没有办法使用 select_related 或
   进行预取，我必须使用 prefetch_related ？


2. 有没有办法预取
   page_set ？这不起作用：

1. Is there a way to do this prefetching using select_related or must I use prefetch_related?
2. Is there a way to prefetch the page_set? This does not work:

Publisher.objects.all（）。prefetch_related（'book_set'，'book_set_page_set '）

推荐答案

1. 不，你不能使用 select_related 为反向关系。 select_related 执行SQL连接，因此主要查询器中的单个记录需要在相关表中引用一个（ ForeignKey 或 OneToOne 字段）。 prefetch_related 实际上完全单独的第二个查询，缓存结果，然后将它加入到python中的查询中。因此，需要 ManyToMany 或反向 ForeignKey 字段。

1. No, you cannot use select_related for a reverse relation. select_related does a SQL join, so a single record in the main queryset needs to reference exactly one in the related table (ForeignKey or OneToOne fields). prefetch_related actually does a totally separate second query, caches the results, then "joins" it into the queryset in python. So it is needed for ManyToMany or reverse ForeignKey fields.

你尝试过两个下划线来做多级预取吗？像这样： Publisher.objects.all（）。prefetch_related（'book_set'，'book_set__page_set'）

Have you tried two underscores to do the multi level prefetches? Like this: Publisher.objects.all().prefetch_related('book_set', 'book_set__page_set')

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