如何获得猫鼬所有集合中两个字段的乘积之和

Question

我有一个商店库存集合，分别包含“数量”和“价格”字段

{
        "_id": "5dd033a1a9133424a8ae2924",
        "name": "Belt",
        "description": "Auto",
        "price": 7,
        "quantity": 60,
        "supplier": "Thomson",
        "taxable": true,
        "createdAt": "2019-11-16T17:36:33.753Z",
        "updatedAt": "2020-01-11T09:33:27.756Z",
        "__v": 0,
        "orders": []
    },

我希望能够获得商店中所有项目的总成本，即所有项目的价格*数量之和

// model

const mongoose= require('mongoose');


//create Schema
const InventorySchema = new mongoose.Schema({
   // _id: mongoose.Schema.Types.ObjectId,
   name : { type: String, required: true },
   description : { type: String, required: true },
   price : { type: Number, required: true },
   quantity : { type: Number, required: true },
   supplier : String,
   taxable : Boolean,
   orders: [{quantity: Number,issuedBy:String,collectedBy:String,department:String}]

},{timestamps:true});

Answer 1

一个简单的$group阶段应该可以解决问题：