Django链prefetch_related和select_related [英] Django chaining prefetch_related and select_related

问题描述

假设我有以下模型

class Foo(models.Model):
    ...

class Prop(models.Model):
    ...

class Bar(models.Model):
    foo: models.ForeignKey(Foo, related_name='bars', ...)
    prop: models.ForeignKey(Prop, ...)

现在我要进行以下查询。

Now I want to make the following query.

foos = Foo.objects.prefetch_related('bars__prop').all()

上面的查询是否进行3次数据库调用或仅进行2次（对于 prop 来自 bar ），因为只有一个 prop 与 bar

Does the above query makes 3 Database calls or only 2 (select_related for prop from bar), given that only one prop is associated with bar

如果需要接听3个电话，是否可以通过对<$ c使用selected_related使其成为2个电话$ c> bar-> prop

If it takes 3 calls then, is there a way to make it 2 calls by using selected_related for bar -> prop

推荐答案

您可以使用 Prefetch 类来指定 prefetch_related（）中使用的查询集，并通过这种方式将其与 select_related（）组合：

You can use the Prefetch class to specify the queryset that is used in prefetch_related() and this way combine it with select_related():

from django.db.models import Prefetch
bars = Bar.objects.select_related('prop')
foos = Foo.objects.prefetch_related(Prefetch('bars', queryset=bars)).all()

这应该是两个查询，一个用于 Foo 对象，一个用于获取相关的 Bar 对象与 Prop 相同的查询。

Note that this should be two queries, one for the Foo objects and one for getting the related Bar objects that are being joined in the same query with Prop.

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