Django根据条件计算相关对象

Question

我想在计数之前过滤结果。在下面的代码中，我正在计算每个票务柜台的工人总数。

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

上述代码中的workers 是来自另一个模型（WorkerToTicketCounter 模型）的related_name。我想要做的是，我希望能够过滤条件为is_deleted= True 的工人，而不是计算所有工人。可能吗？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"))

明白了吗？我想过滤workers。

编辑： 我的模特：

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',
        )

Answer 1

由于django-2.0，您可以在Count 中进行过滤，但这对我们没有帮助。

然而我们可以做的是总结workers__is_deleted，或者它的否定。例如，如果我们想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

或者如果你想统计被删除的工人：

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