nodejs+express+mongodb: error connecting to database

I'm trying to make a microblog web app width nodejs, express and mongodb. But I came up with a problem that my app cannot access to the database.

My package.json goes like these:

    {
    "name": "microblog"
  , "version": "0.0.1"
  , "private": true
  , "dependencies": {
      "express": "2.5.8"
    , "ejs": "0.0.1"
    , "connect-mongo" : "0.1.7"
    , "mongodb" : "0.9.9"
  }
}

Configuration goes like these:

    app.configure(function(){
  app.set('views', __dirname + '/views');
  app.set('view engine', 'ejs');
  app.use(express.bodyParser());
  app.use(express.methodOverride());
  app.use(express.cookieParser());
  app.use(express.session({
            secret: settings.cookieSecret,
            store: new MongoStore({
                    db: settings.db
                })
          }));
  app.use(express.router(routes));
  app.use(express.static(__dirname + '/public'));

});

I create a mongodb with db.js in directory models:

module.exports = {
    cookieSecret: 'microblog',
    db: 'microblog',
    host: 'localhost'
}

With all above I just tried to debug my app to see if the app can successfully connect to the database. Then comes this problem:

/home/edward/github/nodejs/microblog/node_modules/connect-mongo/lib/connect-mongo.js:114
      throw new Error('Error connecting to database');
            ^
Error: Error connecting to database
    at /home/edward/github/nodejs/microblog/node_modules/connect-mongo/lib/connect-mongo.js:114:13
    at /home/edward/github/nodejs/microblog/node_modules/mongodb/lib/mongodb/db.js:240:16
    at null.<anonymous> (/home/edward/github/nodejs/microblog/node_modules/mongodb/lib/mongodb/connection/server.js:390:7)
    at EventEmitter.emit (events.js:95:17)
    at null.<anonymous> (/home/edward/github/nodejs/microblog/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:96:15)
    at EventEmitter.emit (events.js:98:17)
    at Socket.<anonymous> (/home/edward/github/nodejs/microblog/node_modules/mongodb/lib/mongodb/connection/connection.js:388:10)
    at Socket.EventEmitter.emit (events.js:95:17)
    at net.js:441:14
    at process._tickCallback (node.js:415:13)

It sucks me so much time and I just can't find a way to get out of this. Finally I have to post it here. Any guidance are appreciated. :)