django - 我们如何计算django中相关表中日期时间字段的平均差异？

Question

我有两个型号 演出 和 订单 .
并想计算 之间差异的平均值order_start_time 和 订单完成时间每场演出。检查我的代码它给出以下错误
无法将关键字“订单”解析为字段。选项有:category、category_id、details、gig、id、images、price、reviews、seller、seller_id、title
请帮忙!
模型.py (在卖家应用中)

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 self._average_completionTime is not None:
            return self._average_completionTime
        return self.gig.aggregate(Avg('order_completed_time'-'order_start_time'))

我认为这里的问题在于平均完成时间如何在我应该在 views.py 中引用的一个变量中使用 'order_completed_time'-'order_start_time'
模型.py (在买家应用中)
专注于项目领域

from seller.models import Gigs
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')
    order_start_time = models.DateTimeField(default=None)
    order_completed_time = models.DateTimeField(default=None)
    created_at = models.DateTimeField(auto_now_add=True)

Views.py

class RetrieveGigsAPI(GenericAPIView, RetrieveModelMixin):
    def get_queryset(self):
        return Gigs.objects.all().annotate(_average_rating=Avg('orders__time'))
    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']

Answer 1

您可以使用平均完成时间来注释每个演出，如下所示:

    def get_queryset(self):
        return Gigs.objects.annotate(
            _average_completionTime=Avg(
                F('gig__order_completed_time') - F('gig__order_start_time')
            )
        )

在您的 average_completionTime模型方法，你还需要使用 F表达式 if self._average_completionTime不是这样设置的:

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

更新 :
我无法重现 'decimal.Decimal' object has no attribute 'tzinfo'错误。
不知道为什么，但基于 this 回答，你可以试试用 ExpressionWrapper 为了避免同样的错误，所以:

from django.db.models import DurationField, ExpressionWrapper, F


class GigsSerializerWithAvgTime(serializers.ModelSerializer):
    def get_queryset(self):
        return Gigs.objects.annotate(
            _average_completionTime=Avg(
                ExpressionWrapper(F('gig__order_completed_time') - F('gig__order_start_time'), output_field=DurationField())
            )
        )

或者正如@Abdul Aziz Barkat 提到的，添加 output_field到 Avg像这样聚合:

        return Gigs.objects.annotate(
            _average_completionTime=Avg(
                F('gig__order_completed_time') - F('gig__order_start_time'), output_field=DurationField()
            )
        )