Django如何从关联对象累加数值至主对象字段

如何将多个关联对象中字段的整数值累加后，展示到主对象的指定字段中？具体需求为：将客户所有历史订单中的how_many_order字段值累加，输出到Customer对象的all_previous_orders字段中。

预期输出示例 #

[
    {
        "customer_name": "John Doe",
        "orders": [
            {
                "order_name": "fruit",
                "how_many_order": 10
            },
            {
                "order_name": "car",
                "how_many_order": 1
            }
        ],
        "all_previous_orders": 11
    }
]

现有代码 #

Models.py

class Customer(models.Model):
    customer_name = models.CharField(max_length=100)

class Order(models.Model):
    customer = models.ForeignKey(Customer, on_delete=models.CASCADE)
    order_name = models.CharField(max_length=100)
    how_many_order = models.IntegerField(default=0)

Serializers.py

class OrderSerializer(serializers.ModelSerializer):
    class Meta:
        model = Order
        fields = '__all__'

class CustomerSerializer(serializers.ModelSerializer):
    orders = OrderSerializer(many=True, read_only=True, required=False)

    class Meta:
        model = Customer
        fields = '__all__'

方法1：序列化器中自定义字段计算 #

通过SerializerMethodField在序列化阶段直接计算订单总和：

from rest_framework import serializers
from django.db.models import Sum

class CustomerSerializer(serializers.ModelSerializer):
    orders = OrderSerializer(many=True, read_only=True, required=False)
    all_previous_orders = serializers.SerializerMethodField()

    def get_all_previous_orders(self, obj):
        # 数据库层面聚合求和，性能优于Python遍历
        total = obj.order_set.aggregate(total=Sum('how_many_order'))['total']
        return total if total is not None else 0

    class Meta:
        model = Customer
        fields = ['customer_name', 'orders', 'all_previous_orders']

方法2：模型中定义属性字段 #

在Customer模型中新增属性，让模型自身具备计算能力：

修改Models.py #

from django.db.models import Sum

class Customer(models.Model):
    customer_name = models.CharField(max_length=100)

    @property
    def all_previous_orders(self):
        total = self.order_set.aggregate(total=Sum('how_many_order'))['total']
        return total if total is not None else 0

修改Serializers.py #

class CustomerSerializer(serializers.ModelSerializer):
    orders = OrderSerializer(many=True, read_only=True, required=False)
    all_previous_orders = serializers.ReadOnlyField()

    class Meta:
        model = Customer
        fields = ['customer_name', 'orders', 'all_previous_orders']

方法3：批量查询时预计算（性能最优） #

如果需要批量查询客户，在视图中用annotate提前在数据库层面完成计算，减少查询次数：

视图示例 #

from django.db.models import Sum
from rest_framework import generics
from .models import Customer
from .serializers import CustomerSerializer

class CustomerListView(generics.ListAPIView):
    queryset = Customer.objects.annotate(
        all_previous_orders=Sum('order__how_many_order')
    ).all()
    serializer_class = CustomerSerializer

序列化器调整 #

只需确保字段列表包含all_previous_orders：

class CustomerSerializer(serializers.ModelSerializer):
    orders = OrderSerializer(many=True, read_only=True, required=False)

    class Meta:
        model = Customer
        fields = ['customer_name', 'orders', 'all_previous_orders']

内容的提问来源于stack exchange，提问作者mRezaAnanta