不同模型中两个属性之间的算术运算

Question

我想对来自不同模型的两个不同属性应用算术运算，并获取json作为最终结果。例如在这种情况下，我想通过将price_per_km属性乘以第二个对象中的distance属性来计算每辆车的价格

此条目json

{
  "cars": [
    { "id": 1, "price_per_day": 2000, "price_per_km": 10 },
    { "id": 2, "price_per_day": 3000, "price_per_km": 15 },
    { "id": 3, "price_per_day": 1700, "price_per_km": 8 }
  ],
  "rentals": [
    { "id": 1, "car_id": 1, "start_date": "2017-12-8", "end_date": "2017-12-10", "distance": 100 },
    { "id": 2, "car_id": 1, "start_date": "2017-12-14", "end_date": "2017-12-18", "distance": 550 },
    { "id": 3, "car_id": 2, "start_date": "2017-12-8", "end_date": "2017-12-10", "distance": 150 }
  ]
}

预期的json *

{
  "rentals": [
    {
      "id": 1,
      "price": 7000
    },
    {
      "id": 2,
      "price": 15500
    },
    {
      "id": 3,
      "price": 11250
    }
  ]
}

在哪里可以在viewset或serializer.py的queryset中编写sum方法？ 如何自定义我的视图集以获取结果？