Join in node.js with mongodb using nested find

i fetched all rides info from dectar_rides collection of mongodb i have user.id from dectar_rides collection now from this user.id i want to fetch username from dectar_users collection of mmongodb kindly plz help

var express = require('express');
var app = express();
var MongoClient = require('mongodb').MongoClient;
var url = 'mongodb://localhost:27017/uride_mdb';
var str = "";
app.route('/rides').get(function(req, res) {
   MongoClient.connect(url, function(err, db) {
       var collection = db.collection('dectar_rides');
       var cursor = collection.find({}).limit(5);
       str = "";
       cursor.forEach(function(item) {
           if (item != null) {
            var c = 2+2;
            var  userid = item.user.id;
            var usertable = db.collection('dectar_rides');
            var cursorusers = usertable.findOne({_id:item.user.id});
            str = str + "Id " + 
            item.ride_id + '- Location Name ' +
            item.location.name + '- user id ' +
            item.user.id + 
            cursorusers.user_name
            +"</br>";
           }
       }, function(err) {
           res.send(str);
           db.close();
          }
       );
   });
});
var server = app.listen(8080, function() {});