QuerySet by聚合字段值 [英] A QuerySet by aggregate field value

问题描述

假设我有以下模型：

class Contest:
    title = models.CharField( max_length = 200 )
    description = models.TextField()

class Image:
    title = models.CharField( max_length = 200 )
    description = models.TextField()
    contest = models.ForeignKey( Contest )
    user = models.ForeignKey( User )

    def score( self ):
        return self.vote_set.all().aggregate( models.Sum( 'value' ) )[ 'value__sum' ]

class Vote:
    value = models.SmallIntegerField()
    user = models.ForeignKey( User )
    image = models.ForeignKey( Image )

站点的用户可以贡献其图像参加几场比赛。然后其他用户可以对其进行投票。

The users of a site can contribute their images to several contests. Then other users can vote them up or down.

一切正常，但是现在我要显示一个页面，用户可以在其中看到对某项竞赛的所有贡献。图片应按其得分排序。
因此，我尝试了以下操作：

Everything works fine, but now I want to display a page on which users can see all contributions to a certain contest. The images shall be ordered by their score. Therefore I have tried the following:

Contest.objects.get( pk = id ).image_set.order_by( 'score' )

我担心自以来，它不起作用 不是可在查询中使用的数据库字段。

As I feared it doesn't work since 'score' is no database field that could be used in queries.

推荐答案

哦，我当然忘记了关于Django中新的聚合支持及其 annotate 功能。

Oh, of course I forget about new aggregation support in Django and its annotate functionality.

所以查询看起来像这样：

So query may look like this:

Contest.objects.get(pk=id).image_set.annotate(score=Sum('vote__value')).order_by( 'score' )

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