在Django中组合prefetch_related和annotate

Question

我有三个型号

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queryset

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 %}

我没有计算连接到每个ModelB对象的ModelC对象的数量,而是想要对ModelC中的amount字段求和.

我不知道如何结合prefetch_related并annotate在我的查询集,但它必须是这样的

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

Answer 1

我认为您应该能够通过这样做来实现这一目标：

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，您应该使用：

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