Django 根据条件对相关对象进行计数

Question

我想在计数之前过滤结果。在下面的代码中，我计算了每个售票柜台的工作人员总数。

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 是另一个模型(WorkerToTicketCounter 模型)中的 lated_name。我想要做的是，我希望能够使用条件 is_deleted= True 过滤工作人员，而不是计算所有工作人员。是否可以？我正在使用 Django 1.11.13

有没有类似下面的内容

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

明白了吗？我想过滤 worker 。

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',
        )

Answer 1

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

但是，我们可以做的是总结 workers__is_deleted 或其否定。例如，如果我们想返回未删除的工作人员数量:

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=<b>Cast(
            Coalesce(Sum(Value(1) - F('workers__is_deleted')), Value(0))</b>,
            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=<b>Cast(
            Coalesce(Sum(F('workers__is_deleted')), Value(0))</b>,
            IntegerField()
        )
    )
    serializer_class = TicketCounterSerializer