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django - Django根据条件计算相关对象

问题描述

我想在计数之前过滤结果。在下面的代码中，我正在计算每个售票柜台的工人总数。

class TicketCounterList(ListAPIView):
    queryset = TicketCounter.objects.filter(ticket_counter_is_deleted=False)
            .annotate(num_workers=Count('workers'))
    serializer_class = TicketCounterSerializer

workers在上面的代码中是related_name来自另一个模型（WorkerToTicketCounter模型）。我想要做的是，我希望能够过滤有条件的工人，is_deleted= True而不是计算所有工人。是否可以？我正在使用 Django 1.11.13

有没有像下面这样的

queryset = TicketCounter.objects.filter(ticket_counter_is_deleted=False)
            .annotate(num_workers=Count('workers',filter="xxxx"))

得到它？我想过滤workers.

编辑：我的模型：

class TicketCounter(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    ticket_counter_name = models.CharField(max_length=100, default="")
    ticket_counter_description = models.CharField(max_length=1500, default="")
    ticket_counter_address = models.CharField(max_length=1500, default="")


class WorkerToTicketCounter(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    ticket_counter = models.ForeignKey(TicketCounter,related_name="workers")
    worker = models.ForeignKey(User,related_name='worker_for_ticket_counter')

我的序列化器：

class TicketCounterSerializer(serializers.ModelSerializer):
    num_workers = serializers.IntegerField()
    class Meta:
        model = TicketCounter
        fields = (
            'ticket_counter_name',
            'ticket_counter_description',
            'ticket_counter_address',
            'num_workers',
        )

标签： djangodjango-modelsdjango-rest-framework

解决方案

从，您可以在 中进行过滤Count，但这对我们没有帮助。

然而，我们可以做的是总结，workers__is_deleted或它的否定。例如，如果我们要返回未删除的工人数量：

from django.db.models import F, IntegerField, Sum, Value
from django.db.models.functions import Coalesce

class TicketCounterList(ListAPIView):
    queryset = TicketCounter.objects.filter(
        ticket_counter_is_deleted=False
    ).annotate(
        num_workers=Cast(
            Coalesce(Sum(Value(1) - F('workers__is_deleted')), Value(0)),
            IntegerField()
        )
    )
    serializer_class = TicketCounterSerializer

或者，如果您想计算已删除的工作人员：

from django.db.models import F, IntegerField, Sum, Value
from django.db.models.functions import Coalesce

class TicketCounterList(ListAPIView):
    queryset = TicketCounter.objects.filter(
        ticket_counter_is_deleted=False
    ).annotate(
        num_workers=Cast(
            Coalesce(Sum(F('workers__is_deleted')), Value(0)),
            IntegerField()
        )
    )
    serializer_class = TicketCounterSerializer

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