问当需要从另一个文件中的模式时，猫鼬填充未定义的返回

EN

var mongoose = require("mongoose");
var Schema = mongoose.Schema;//creating schema


var ListingSchema = new Schema({
location: {//ROOM LOCATION
            type: [Number],  // [<longitude>, <latitude>]
            index: '2d'      // create the geospatial index
    },
}
);


var Listing = mongoose.model('Listing', ListingSchema);

module.exports = Listing;

var mongoose = require("mongoose");
var Schema = mongoose.Schema;//creating schema
var Listing=require('../listing/listingModel');


var UserSchema = new Schema({

favoriteListings :  [{ type: Schema.Types.ObjectId, ref: 'Listing' }],


}

);


var User = mongoose.model('User', UserSchema);


module.exports = User;

addFavListing:function(req,res){

//READ TOKEN AND FIND USER ID, IF USER HAS REACHED THIS POINT, THIS MEANS THAT TOKEN ALREADY
//HAS BEEN VERIFIED
var token = req.body.token || req.query.token || req.headers['x-access-token'];
var decoded=jwt.verify(token, app.get('superSecret'));
var id=decoded._doc._id;console.log(id);


User.find({_id:id}).populate('favoriteListings').exec(function(err,users) {
  if (err){ return handleError(err);}
    console.log(users.favoriteListings);

});