How can I require the module into my file?

I create an express project and the directory structure like:

/
  - model
    db.js
  - routes
    users.js
  app.js

In ./model/db.js, I have a MySql connection:

var mysql      = require('mysql');
var connection = mysql.createConnection({
host     : 'localhost',
user     : 'root',
password : 'root',
database : 'imei_node'
});

connection.connect();

And in ./routes/users.js, I need to query database:

var express = require('express');
var router = express.Router();
var db = require('./model/db.js');   // always cannot find the module....

router.route('/')
.get(function (req, res) {
    connection.query(
        'select * from user',
        function (err, rows, fields) {
            if (err) {
                res.status(500).send('error');
            } else {
                res.send({
                    result      : 'success',
                    err         : '',
                    err_type    : '',
                    fields      : fields,
                    rows        : rows,
                    length      : rows.length
                });
            }
        }
    )
});


module.exports = router;

But the debugger always says that Cannot find the module './model/db.js'.

I am new to nodejs, can anybody tell me how to require the db.js into routes file? Thanks a lot.