Django REST Framework 序列化程序中的嵌套注释字段

Question

我正在尝试查看 Django REST Framework 序列化程序中的嵌套注释（聚合/计算）字段。这将允许更干净地处理带注释的字段。这篇文章与Aggregate (and other annotated) fields in Django Rest Framework serializers 类似，但是我想要一种类似的嵌套技术。在方法下方可以看到如何在没有嵌套的情况下工作，以及如何在嵌套情况下不工作。

我知道这可以手动实现（使用 Django 视图）或通过使用使我不感兴趣的数据库重载的方法来实现。但也许有一个高性能且优雅的解决方案来解决这个问题。

以下作品（未嵌套）

模型

class IceCreamCompany(models.Model):
    name = models.CharField(max_length=255)


class IceCreamTruck(models.Model):
    company = models.ForeignKey('IceCreamCompany', related_name='trucks')
    capacity = models.IntegerField()


class IceCreamTruckDriver(models.Model):
    name = models.CharField(max_length=255)
    first_name = models.CharField(max_length=255)
    truck = models.ForeignKey('IceCreamTruck', related_name='drivers')

序列化器

class IceCreamTruckDriverSerializer(serializers.ModelSerializer):

    class Meta:
        model = IceCreamTruckDriver
        fields = ('name', 'first_name')


class IceCreamTruckSerializer(serializers.ModelSerializer):
    drivers = IceCreamTruckDriverSerializer(many=True, read_only=True)

    class Meta:
        model = IceCreamTruck
        fields = ('capacity', 'drivers')


class IceCreamCompanySerializer(serializers.ModelSerializer):
    trucks = IceCreamTruckSerializer(many=True, read_only=True)
    amount_of_trucks = serializers.IntegerField()

    class Meta:
        model = IceCreamCompany
        fields = ('name', 'trucks', 'amount_of_trucks')

视图集

class IceCreamCompanyViewSet(viewsets.ModelViewSet):
    queryset = IceCreamCompany.objects.prefetch_related('trucks', 'trucks__drivers')\
                           .annotate(amount_of_trucks=Count('trucks'))\
                           .all()

    serializer_class = IceCreamCompanySerializer

结果

"results": [
        {
            "name": "Pete Ice Cream",
            "trucks": [
                {
                    "capacity": 35,
                    "drivers": [
                        {
                            "name": "Damian",
                            "first_name": "Ashley"
                        },
                        {
                            "name": "Wilfrid",
                            "first_name": "Lesley"
                        }
                    ]
                },
                {
                    "capacity": 30,
                    "drivers": [
                        {
                            "name": "Stevens",
                            "first_name": "Joseph"
                        }
                    ]
                },
                {
                    "capacity": 30,
                    "drivers": []
                }
            ],
            "amount_of_trucks": 3
        }
    ]

以下不起作用（嵌套）

同款

序列化器

class IceCreamTruckDriverSerializer(serializers.ModelSerializer):

    class Meta:
        model = IceCreamTruckDriver
        fields = ('name', 'first_name')


class IceCreamTruckSerializer(serializers.ModelSerializer):
    drivers = IceCreamTruckDriverSerializer(many=True, read_only=True)
    amount_of_drivers = serializers.IntegerField()

    class Meta:
        model = IceCreamTruck
        fields = ('capacity', 'drivers', 'amount_of_drivers')


class IceCreamCompanySerializer(serializers.ModelSerializer):
    trucks = IceCreamTruckSerializer(many=True, read_only=True)

    class Meta:
        model = IceCreamCompany
        fields = ('name', 'trucks')

视图集

class IceCreamCompanyViewSet(viewsets.ModelViewSet):
    queryset = IceCreamCompany.objects.prefetch_related('trucks', 'trucks__drivers')\
                           .annotate(trucks__amount_of_drivers=Count('trucks__drivers'))\
                           .all()

    serializer_class = IceCreamCompanySerializer

结果

AttributeError at /ice/
Got AttributeError when attempting to get a value for field `amount_of_drivers` on serializer `IceCreamTruckSerializer`.
The serializer field might be named incorrectly and not match any attribute or key on the `IceCreamTruck` instance.
Original exception text was: 'IceCreamTruck' object has no attribute 'amount_of_drivers'.

Answer 1

作为参考，还可以在模型 IceCreamTruck 上注释每辆卡车的司机数量，例如使用自定义管理器：

class AnnotatedManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().annotate(amount_of_drivers=Count('drivers'))

class IceCreamTruck(models.Model):
    company = models.ForeignKey('IceCreamCompany', related_name='trucks')
    capacity = models.IntegerField()

    objects = AnnotatedManager()

那么您不需要注释视图集，因为amount_of_drivers 已经在trucks 上注释：

class IceCreamCompanyViewSet(viewsets.ModelViewSet):
    queryset = IceCreamCompany.objects.prefetch_related('trucks', 'trucks__drivers').all()
    serializer_class = IceCreamCompanySerializer 

它应该比在序列化器中计数更有效。

Answer 2

我使用 Django REST google 组在 IntegerField 中使用 read_only=True 得到了答案，这有助于消除错误，但该字段不再显示。也许我的注释是错误的。无论如何，我最终在 Django 中使用了自定义视图，因为我最终需要更多数据。但是您可以通过其他方式获取数据：

一个非常优雅的解决方案是删除注释函数并使用可以给我结果的 SerializerMethodField。

但是：这确实会引起很多查询！

同款

序列化器

class IceCreamTruckDriverSerializer(serializers.ModelSerializer):

    class Meta:
        model = IceCreamTruckDriver
        fields = ('name', 'first_name')


class IceCreamTruckSerializer(serializers.ModelSerializer):
    drivers = IceCreamTruckDriverSerializer(many=True, read_only=True)
    amount_of_drivers = serializers.SerializerMethodField()

    def get_amount_of_drivers(self, obj):
        return obj.drivers.count()

    class Meta:
        model = IceCreamTruck
        fields = ('capacity', 'drivers', 'amount_of_drivers')


class IceCreamCompanySerializer(serializers.ModelSerializer):
    trucks = IceCreamTruckSerializer(many=True, read_only=True)

    class Meta:
        model = IceCreamCompany
        fields = ('name', 'trucks')

视图集

class IceCreamCompanyViewSet(viewsets.ModelViewSet):
    queryset = IceCreamCompany.objects.prefetch_related('trucks', 'trucks__drivers').all()

    serializer_class = IceCreamCompanySerializer

结果

"results": [
        {
            "name": "Pete Ice Cream",
            "trucks": [
                {
                    "capacity": 35,
                    "drivers": [
                        {
                            "name": "Damian",
                            "first_name": "Ashley"
                        },
                        {
                            "name": "Wilfrid",
                            "first_name": "Lesley"
                        }
                    ],
                    "amount_of_drivers": 2
                },
                {
                    "capacity": 30,
                    "drivers": [
                        {
                            "name": "Stevens",
                            "first_name": "Joseph"
                        }
                    ],
                    "amount_of_drivers": 1
                },
                {
                    "capacity": 30,
                    "drivers": [],
                    "amount_of_drivers": 0
                }
            ]
        }
    ]

也可以像这样在模型中使用函数：Django Rest Framework Ordering on a SerializerMethodField（它在代码本身中可见）但我没有选择它，所以我不必过多地修改我的模型。这也会产生过多的查询。