django.core.exceptions.FieldError：尝试选择属性字段时无法将关键字“持续时间”解析为字段

Question

关于这个特定错误有很多问题，但没有一个是由于尝试从序列化程序中选择属性字段引起的，下面是我的代码

 # seriliazer.py
    class ProjectActivitySerializer(serializers.ModelSerializer):
        is_running = serializers.ReadOnlyField()
        duration = serializers.ReadOnlyField() # i am trying to select this particular field to perform some calculation
        
        class Meta:
            model = ProjectActivity
            fields = '__all__'


 class ProjectActivity(models.Model):
    id = models.AutoField(primary_key=True)
    description = models.TextField(blank=False, null=False, 
     max_length=400, default="")
    project = models.ForeignKey(to=Project, 
    on_delete=models.CASCADE)
    user = models.ForeignKey(to=Employee, on_delete=models.CASCADE)
    start_time = models.DateTimeField(auto_now_add=True)
    end_time = models.DateTimeField(blank=True, null=True)

    @property
    def duration(self):
        """
        get the duration of the activity

        Returns:
            timedelta: duration of the activity
        """
        if self.is_running:
            sec = datetime.now(timezone.utc) - self.start_time
            return str(timedelta(seconds=round(sec.total_seconds())))
        else:
            sec = self.end_time - self.start_time
            return str(timedelta(seconds=round(sec.total_seconds())))
          

    class GetTotalProjectActivityTime(ListAPIView):
        """
        
        """
        serializer_class = ProjectActivitySerializer
        permission_classes = (IsAuthenticated, IsOwner|IsAdminUser) # protect the endpoint
    
    
        def get_queryset(self):
            return ProjectActivity.objects.filter(user=self.kwargs['user'], project__id=self.kwargs['project']).values('duration')
            

Just a background to my problem here: i have the below response

    [
        {
            "id": 1,
            "is_running": true,
            "duration": "2 days, 5:43:26",
            "description": "worked on developing dashboard endpoint",
            "start_time": "2021-12-22T11:40:49.452935Z",
            "end_time": null,
            "project": 1,
            "user": 1
        }
    ]

我想对持续时间进行计算，但持续时间字段不是我的模型字段的一部分，而是一个公正的属性。持续时间值是从模型的持续时间属性中获取的，我想从响应中计算总持续时间。

现在我发现运行计算很困难

Answer 1

首先将duration 属性更改为使用cached_property，这样做可以让您创建具有相同名称的注释，并且在对象上设置注释值时不会发生冲突。

from django.utils.functional import cached_property
from django.utils.timezone import now

class ProjectActivity(models.Model):

    @cached_property
    def duration(self):
        return (self.end_time or now()) - self.start_time

现在我们可以使用以下方法用 duration 注释查询集

from django.db.models import F
from django.db.models.functions import Coalesce, Now

queryset = ProjectActivity.objects.annotate(
    end_or_now=Coalesce('end_time', Now()))
).annotate(
    duration=F('end_or_now') - F('start_time')
)

然后可以在进一步的注释、过滤器、聚合等中使用此注释

from django.db.models import Sum

queryset.aggregate(total_duration=Sum('duration'))