如何在 django 中计算某些 if 条件的字段平均值？

Posted 2023-02-23

【中文标题】如何在 django 中计算某些 if 条件的字段平均值？

【英文标题】：How to calculate average of fields on some if conditions in django?

【发布时间】：2021-11-20 19:06:15

【问题描述】：

我在 Django 上工作，我有两个模型 Gigs 和 Orders，我正在计算每个 gig 的订单的平均完成时间。

在 订单 模型中，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isCompleted = True

的那些

订单

的平均值

Models.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那些 iscompleted 为 True 的订单完成时间的平均值

【参考方案1】：

您可以为Avg 指定一个过滤器，以仅聚合基于isCompleted 的已完成订单，如下所示：

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 
            )
        )

【参考方案2】：

    if isCompleted:
        foo = Gigs.objects.annotate(_average_completionTime=Avg(
                ExpressionWrapper(F('gig__orderCompletedTime') F('gig__orderStartTime'), output_field=DurationField())
            )
        )

        return foo