我有学生收藏和课程收藏
student._id(ObjectId)需要匹配classes.owner(String)上的文件集合的查找.我正在获取文件的空数组.我究竟做错了什么？

var express = require('express');
var router = express.Router();
var Account = require('../models/account');
var Connections = require('../models/connections');
var mongoose = require('mongoose');
var Schema = mongoose.Schema;


//get all advocates for student.
router.route('/:student_id?')

  .get(function (req, res) {

    Account.aggregate(
      { "$match": { "_id": mongoose.Types.ObjectId(req.params.student_id) } },
      {$lookup:
        {
          from: 'classes',
          localField: 'owner',//**String**
          foreignField: '_id', //**ObjectId**
          as: 'classes'
        }
      }       
    ).exec(function (err, doc) {
        console.log(doc[0]);
        if (err) {
          res.send(err);
        } else {
          res.send(doc[0]).end();
        }
      })
  });


module.exports = router;

解决方法:

检查你的条款,这是你的收藏品名称？我使用模式名称而不是集合名称导致空结果.

{$lookup:
        {
          from: 'classes',  //check this
          localField: 'owner',//**String**
          foreignField: '_id', //**ObjectId**
          as: 'classes'
        }
      }