django - 如何计算django中某些if条件的字段平均值？

Question

我在 Django 上工作，我有两个模型 演出 和 订单我正在计算每个演出订单的平均完成时间。
在 订购 模型我有两个字段订单开始时间 (每当卖家接受订单时我都会发送)和 订单完成时间 (我在卖家发货时发送)订单。
但我只想计算那些 的平均值订单哪里isCompleted = True 模型.py

class Orders(models.Model):
    buyer = models.ForeignKey(User,default=None, on_delete=models.CASCADE,related_name='buyer_id')
    seller = models.ForeignKey(User,default=None, on_delete=models.CASCADE,related_name='seller_id')
    item = models.ForeignKey(Gigs,default=None, on_delete=models.CASCADE,related_name='gig')
    payment_method= models.CharField(max_length=10)
    address = models.CharField(max_length=255)
    mobile = models.CharField(max_length=13,default=None)
    quantity = models.SmallIntegerField(default=1)
    status = models.CharField(max_length=13,default='new order')
    orderStartTime = models.DateTimeField(default=timezone.now)
    orderCompletedTime = models.DateTimeField(default=timezone.now)
    isCompleted = models.BooleanField(default=False)
    created_at = models.DateTimeField(auto_now_add=True)


class Gigs(models.Model):
    title = models.CharField(max_length=255)
    category = models.ForeignKey(Categories , on_delete=models.CASCADE)
    images = models.ImageField(blank=True, null = True, upload_to= upload_path)
    price = models.DecimalField(max_digits=6, decimal_places=2)
    details = models.TextField()
    seller = models.ForeignKey(User,default=None, on_delete=models.CASCADE)

    @property
    def average_completionTime(self):
        if getattr(self, '_average_completionTime', None):
            return self._average_completionTime
        return self.gig.aggregate(Avg(F('orderCompletedTime') - F('orderStartTime')))

Views.py

class RetrieveGigsAPI(GenericAPIView, RetrieveModelMixin):
    def get_queryset(self):
        return Gigs.objects.annotate(
            _average_completionTime=Avg(
                ExpressionWrapper(F('gig__orderCompletedTime') - F('gig__orderStartTime'), output_field=DurationField())
            )
        )
    serializer_class = GigsSerializerWithAvgTime
    permission_classes = (AllowAny,)

    def get(self, request , *args, **kwargs):
        return self.retrieve(request, *args, **kwargs)

Serializers.py

class GigsSerializerWithAvgTime(serializers.ModelSerializer):
    average_completionTime = serializers.SerializerMethodField()
    def get_average_completionTime(self, obj):
        return obj.average_completionTime
    class Meta:
        model = Gigs
        fields = ['id','title','category','price','details','seller','images','average_completionTime']

请告诉我如何才能获得 iscompleted 为 True 的那些订单完成时间的平均值

Answer 1

您可以为 Avg 指定过滤器仅根据 isCompleted 汇总已完成的订单像这样:

class RetrieveGigsAPI(GenericAPIView, RetrieveModelMixin):
    def get_queryset(self):
        return Gigs.objects.annotate(
            _average_completionTime=Avg(
                ExpressionWrapper(F('gig__orderCompletedTime') - F('gig__orderStartTime'), output_field=DurationField()),
                filter=Q(gig__isCompleted=True),
                # ^^^ Add this 
            )
        )