nodejs get all collection from mongodb database

I've defined simple connection to mongo db to fetch all saved records into database and show I get null and my findAll does not work:

var Db = require('mongodb').Db;
var Connection = require('mongodb').Connection;
var Server = require('mongodb').Server;

ArticleProvider = function (host, port) {
    this.db = new Db('article', new Server(host, port, {auto_reconnect: true}), {safe:false});
    this.db.open(function () {
    });
};

ArticleProvider.prototype.getCollection = function (callback) {
    this.db.collection('article'), function (error, article_collection) {
        if (error) callback(error);
        else callback(null, article_collection);
    }
};

ArticleProvider.prototype.findAll = function(callback) {
    this.getCollection(function(error, article_collection) {
        if( error ) callback(error)
        else {
            article_collection.find().toArray(function(error, results) {
                if( error ) callback(error)
                else callback(null, results)
            });
        }
    });
};

exports.ArticleProvider = ArticleProvider;

for get all saved data in article database, i'm testing log and find all saved records into article from mongo shell:

> db.article.find()
{ "_id" : ObjectId("55033f527be6c3a7bc7e5b04"), "title" : "Post two", "body" : "Body two" }
{ "_id" : ObjectId("55033f527be6c3a7bc7e5b05"), "title" : "Post three", "body" : "Body three" }
> 

in my nodejs Route i get null and i can not get this records from my application

My nodejs server:

var express = require('express');
var app = express();
var server = require('http').createServer(app);
var io = require('../')(server);
var connect = require('connect');
app.use(connect.logger('dev'));
app.use(connect.json());  
app.use(connect.urlencoded());


require('../routes/routes.js')(app);

var port = process.env.PORT || 3000;
server.listen(port, function () {
  console.log('Server listening at port %d', port);
});

My Route content file:

var ArticleProvider = require('config/models/connection').ArticleProvider;
var articleProvider = new ArticleProvider('localhost', 27017);

module.exports = function (app) {
    app.get('/', function (req, res) {
        articleProvider.findAll( function(error,docs){
            console.log(docs);
        });
        res.end();
    });

};