如何检查mongo数据库是否已连接？

Question

我有mongodb启动并运行应用程序如何确保应用程序启动时它与db连接？

app.js

var express        = require('express');
var app            = express();
var bodyParser     = require('body-parser');
var methodOverride = require('method-override');
var mongoose = require('mongoose');
console.log(mongoose.connection.readyState);
var db = require('./config/db');
var port = process.env.PORT || 8080; 
mongoose.connect(db.url); 
app.use(methodOverride('X-HTTP-Method-Override')); 
app.use(express.static(__dirname + '/public')); 

require('./app/routes')(app); // configure our routes
app.listen(port);                                  
console.log('listening on port ' + port);          
exports = module.exports = app;

config＆gt; db.js

module.exports = {
        url : 'mongodb://localhost/test-dev'
    }