django - 按计算字段排序查询 [英] django - ordering queryset by a calculated field

问题描述

我想要一个具有计算字段的模型，我可以应用排序。例如，假设我有以下模型：

I want to have a model with calculated fields that I can apply sorting on. For example, let's say that I have the following model:

class Foo(models.Model):
    A = models.IntegerField(..)
    B = models.IntegerField(..)
    C = models.ForeignKey(..)

我想要一个由以下公式计算的D和E字段：

I want to have a D and an E field that are calculated by the following formulas:

1. D = A - B


2. E = A - X（其中X是C型相关记录的一个字段）

如果我不需要应用排序，实现这一点将是微不足道的;我只是添加属性到模型类。然而，我需要这些字段的排序。

Implementing this would be trivial if I didn't need to apply sorting; I would just add properties to the model class. However, I need ordering by these fields.

一个解决方案是将所有记录都记录到内存中，然后进行排序，这是我最后设想的（它会打破事情有关分页）。

A solution is to fetch all records into memory and do the sorting there, which I conceive a last resort (it will break things regarding pagination).

有没有办法实现我正在尝试？任何指导都不胜感激。

Is there a way to achieve what I'm trying? Any guidance is appreciated.

编辑：字段X的值非常频繁地变化，并且大量的Foo记录与模型C的一个记录相关.X的更新将需要数以千计的E更新。

Denormalization is a no-go. The value of field X changes very frequently and a lot of Foo records are related to one record of model C. An update of X will require thousands of updates of E.

推荐答案

我将看看 extra 方法，并指定 order_by 参数。

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