./model/db.js

// 引入mongoose
const mongoose = require("mongoose");

//解决index索引引起的弃用提醒：(node:13616) DeprecationWarning: collection.ensureIndex is deprecated. Use createIndexes instead.
mongoose.set("useCreateIndex", true);

// 连接数据库
mongoose.connect("mongodb://127.0.0.1:27017/jtdb", { useNewUrlParser: true, useUnifiedTopology: true }, function (err, connection) {
  if (err) {
    console.log(err);
    return;
  }
  console.log("连接数据库成功");
});
module.exports = mongoose;

 

./model/order_item.js

const mongoose = require("./db.js");

var OrderItemSchema = mongoose.Schema({
  order_id: String,
  title: String,
  price: Number,
  num: Number,
});

module.exports = mongoose.model("Order_item", OrderItemSchema, "order_item");

 

order_item.js

/*查询order_item,找出商品名称是牛奶的商品，根据牛奶的订单号，再次查询对应订单的总价格*/
const mongoose = require("mongoose");
var OrderItemModel = require("./model/order_item.js");
var OrderModel = require("./model/order.js");
/*
// 第一种实现方式
OrderItemModel.find(
  {
    _id: "60053a31e811c2445c3e43ea",
  },
  function (err, orderItem) {
    // 先将对象转换为JSON格式【JSON.stringify(对象)】，之后又将格式重新转换为对象【JSON.parse(JSON格式文本)】，得到一个新的对象，从而实现深拷贝
    var order_item = JSON.parse(JSON.stringify(orderItem));//深拷贝
    if (err) return console.log(err);
    var order_id = order_item[0].order_id;
    OrderModel.find({ order_id: order_id }, function (err, order) {
      if (err) return console.log(err);
      order_item[0].order_info = order[0];
      console.log(order_item);
    });
  }
);
 */

//  第二种实现方式,aggregate聚合管道，用的语法还是原生的mongoDB语法

OrderItemModel.aggregate(
  [
    {
      $lookup: {
        from: "order",
        localField: "order_id",
        foreignField: "order_id",
        as: "order_info",
      },
    },
    {
      $match: {
        _id: mongoose.Types.ObjectId("60053a31e811c2445c3e43ea"),
      },
    },
  ],
  function (err, docs) {
    if (err) return console.log(err);
    // ------------------------------------------------------------------------------------------------
    console.log(docs[0]);
    // 输出以下：
    /*
    {
      _id: 60053a31e811c2445c3e43ea,
      order_id: '2',
      title: '金典牛奶',
      price: 10,
      num: 4,
      order_info: [
        {
          _id: 60053a31e811c2445c3e43e2,
          order_id: '2',
          uid: '7',
          trade_no: '222',
          all_price: 140,
          all_num: 4
        }
      ]
    }

     */
    // ------------------------------------------------------------------------------------------------
    // console.log(docs);
    // 输出以下----------
    /*
     [ {
        _id: 60053a31e811c2445c3e43ea,
        order_id: '2',
        title: '金典牛奶',
        price: 10,
        num: 4,
        order_info: [ [Object] ]
      }
    ]
     */

    // console.log(JSON.stringify(docs));//直接输出JSON格式文本--------------
    /*
[{"_id":"60053a31e811c2445c3e43ea","order_id":"2","title":"金典牛奶","price":10,"num":4,"order_info":[{"_id":"60053a31e811c2445c3e43e2","order_id":"2","uid":"7","trade_no":"222","all_price":140,"all_num":4}]}]
     */

    // -------------------------------------------------------------------------------------------
    // var order_info = docs[0].order_info;
    // console.log(order_info);
    // 输出以下： order_info
    /*
    [
      {
        _id: 60053a31e811c2445c3e43e2,
        order_id: '2',
        uid: '7',
        trade_no: '222',
        all_price: 140,
        all_num: 4
      }
    ]
    */
    //  ------------------------------------------------------------------------------------------
  }
);

//  ObjectId("60053a31e811c2445c3e43ec");

 

执行 node ./order_item.js

输出：

{
  _id: 60053a31e811c2445c3e43ea,
  order_id: '2',
  title: '金典牛奶',
  price: 10,
  num: 4,
  order_info: [
    {
      _id: 60053a31e811c2445c3e43e2,
      order_id: '2',
      uid: '7',
      trade_no: '222',
      all_price: 140,
      all_num: 4
    }
  ]
}