猫鼬，使用与运算进行选择以填充

Question

用户架构：

var mongoose                = require("mongoose");
var passportlocalmongoose   = require("passport-local-mongoose");

var userschema = new mongoose.Schema({
   username:{
    type:String,
    unique:true,
    required:true,
   },
  email:{
    type:String,
    unique:true,
    require:true,
  },
  phonenumber:{
    type:String,
    required:true
  },
  password:{
    type:String,
    required:true
  },
  college:{
    type:String,
    required:true
  },
  papers : [
      {
        type:mongoose.Schema.Types.ObjectId,
        ref:"Faper"
      }
  ]

});

userschema.plugin(passportlocalmongoose);

module.exports = mongoose.model("User", userschema);

烟斗模式：

    var mongoose = require("mongoose");

    var faperschema = new mongoose.Schema({

        department:{
            type:String,
            require:true
        },
        subject:{
            type:String,
            required:true
        }
    });


    module.exports = mongoose.model("Faper",faperschema);

如何查询以获取具有给定“主题”的“用户”

这是我的代码的样子

app.get("/findfaper",function(req,res){

   //stack overflow and mongoose doc comes to rescue
   var college = req.query.college;
   var subject = req.query.subject;

   User.
   find({college:college}).populate({
            path: 'papers',
            match: { subject: { $eq: subject }},
        }).
        exec(function(err,founddata){
        if(err){
            console.log("error in finding data");
        }
        else{
            console.log(founddata);
            res.render("fapers",{fapers:founddata});
        }
    });

});

（有时）返回空文件的“用户”

[ { papers: [],
    _id: 5b5f2b278f0c621d3bb32f21,
    username: 'hello',
    college: 'Indian Institute of Technology Delhi',
    email: 'sas@gmail.com',
    phonenumber: 'sasa',
    __v: 1 } ]

有时候我得到的用户没有给定的主题，这是因为

  

User.find（{college：college}）

部分。 那么我如何查询获得大学和论文的“用户”呢？