mongoose的 .save方法如何解决 race condition的问题？

Question

token.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var TokenSchema = new Schema({
    nickname: String,
    token: {
        type: String,
        unique: true
    }
}, {
    collection: 'tokens'
});

var Token = mongoose.model('Token', TokenSchema);
module.exports = Token;

login.js

var crypto = require('crypto');
var express = require('express');
var jwt = require('jsonwebtoken');
var router = express.Router();
var User = require('../modules/user');
var Token = require('../modules/token');

router.get('/', function(req, res) {
    res.render('login', {
        title: '登录界面'
    });
});

router.post('/', function(req, res) {
    User.findOne({
            tel: req.body.tel,
        })
        .exec(function(err, user) {
            var md5 = crypto.createHash('md5'),
                password = md5.update(req.body.password)
                .digest('hex');
            if (!user) {
                return res.json({
                    success: false,
                    message: '认证失败，用户名找不到'
                });
            } else if (user.hash_password === password) {
                return;
            } else {
                return res.json({
                    status: 007,
                    success: false,
                    message: '没有当前用户'
                });
            }
        });
    var tel = req.body.tel;
    var user = new User({
        tel: tel
    });
    var md5 = crypto.createHash('md5'),
        password = md5.update(req.body.password)
        .digest('hex');

    user.set("hash_password", password);
    user.set("name", req.body.name);
    user.set("created_at", Date.now());

    var token = jwt.sign(user, 'whuteditor', {
        expiresIn: "30 days" // 设置过期时间
    });

    var tokenUser = new Token({
        token: token
    });

    tokenUser.set("nickname", req.body.name);

    tokenUser.save(function(err, token) {
        if (err) {
            return res.json({
                status: 120,
                success: false,
                message: "Bad Storage"
            });
        } else {
            res.json({
                status: 100,
                success: true,
                message: 'Enjoy your token!',
                token: token
            });
        }
    });
});

module.exports = router;