如何在Django中过滤对象的计数注解？ [英] How to filter objects for count annotation in Django?

问题描述

考虑简单的Django模型事件和参与者：

Consider simple Django models Event and Participant:

class Event(models.Model):
    title = models.CharField(max_length=100)

class Participant(models.Model):
    event = models.ForeignKey(Event, db_index=True)
    is_paid = models.BooleanField(default=False, db_index=True)

可以轻松地向参与者总数注释事件查询：

It's easy to annotate events query with total number of participants:

events = Event.objects.all().annotate(participants=models.Count('participant'))

如果用过滤的参与者的注释如何注释is_paid = True ？

How to annotate with count of participants filtered by is_paid=True?

我需要无论参与者人数如何，查询所有事件我不需要通过注释结果进行过滤。如果有 0 参与者，那没关系，我只需要注释价值 0

I need to query all events regardless of number of participants, e.g. I don't need to filter by annotated result. If there are 0 participants, that's ok, I just need 0 in annotated value.

示例来自文档在这里不起作用，因为它不包括查询中的对象，而不是使用 0 注释它们。

The example from documentation doesn't work here, because it excludes objects from query instead of annotating them with 0.

更新。 Django 1.8新增了条件表达式功能，所以现在我们可以这样做：

Update. Django 1.8 has new conditional expressions feature, so now we can do like this:

events = Event.objects.all().annotate(paid_participants=models.Sum(
    models.Case(
        models.When(participant__is_paid=True, then=1),
        default=0,
        output_field=models.IntegerField()
    )))

推荐答案

刚刚发现Django 1.8有新的< a href =https://docs.djangoproject.com/en/1.10 / ref / models / conditional-expressions />条件表达式功能，所以现在我们可以这样做：

Just discovered that Django 1.8 has new conditional expressions feature, so now we can do like this:

events = Event.objects.all().annotate(paid_participants=models.Sum(
    models.Case(
        models.When(participant__is_paid=True, then=1),
        default=0, output_field=models.IntegerField()
    )))

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