在Django中结合prefetch_related和批注 [英] Combine prefetch_related and annotate in Django

问题描述

我有三个模型

class ModelA(models.Model):
    name = CharField(max_length=100)

class ModelB(models.Model):
    modela = ForeignKey(ModelA)

class ModelC(models.Model):
    modelb = ForeignKey(ModelB)
    amount = IntegerField()

我可以得到输出

name, number of model c objects
==============
Some name, 312
Another name, 17

使用查询集

ModelA.objects.all().prefetch_related('modelb_set', 'groupb_set__modelc_set')

和模板

{% for modela in modela_list %}
    {% for modelb in modela.modelb_set.all %}
        {{ modelb }}, {{ modelb.modelc_set.count }}
    {% endfor %}
{% endfor %}

计算连接到每个ModelB对象的ModelC对象的数量，我想对ModelC中的数量字段求和。

Instead of counting the number of ModelC objects connected to each ModelB object I want to sum the amount field in ModelC.

我不知道如何将 prefetch_related 和注释结合在一起queryset，但必须类似于

I don't know how to combine prefetch_related and annotate in my queryset, but it must be something like

(ModelA.objects.all()
       .prefetch_related('modelb_set', 'groupb_set__modelc_set')
       .annotate(total_amount=Sum('modelc_set__amount')))

推荐答案

我认为您应该可以通过以下操作实现这一目标：

I think you should be able to achieve that by doing this:

from django.db.models import F, Sum

(ModelA.objects.all()
    .prefetch_related('modelb_set', 'modelb__modelc_set')\
    .values('name')\ # group values by modela.name, read: https://docs.djangoproject.com/en/1.9/topics/db/aggregation/
    .annotate(name = F('name'),
              total_amount = Sum('modelb__modelc__amount')))

，在您的模板中应使用：

and in your template you should use:

{% for modela in modela_list %}
    {{ modela.name }}, {{ modela.total_amount }}
{% endfor %}

这篇关于在Django中结合prefetch_related和批注的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！