app2.js

var express = require('express');

var app = express.createServer();

app.configure(function(){
  app.set('view engine', 'jade');
  app.set('views', __dirname + '/views');
  app.use(express.methodOverride());
  app.use(express.bodyDecoder());
  app.use(express.logger());
  app.use(app.router);
  app.use(express.compiler({ src: __dirname + '/views', enable: ['sass'] }));
});

var ArticleProvider = require('./articleprovider-memory').ArticleProvider;

app.get('/', function(req,res){
  articleProvider.findAll(function(error, docs){
    res.render('blogs_index.jade', {
      locals: {
        title: 'Blog',
        articles: docs
      }
    });
  })
})

app.get('/*.css', function(req,res){
  res.render(req.params[0] + '.css.sass', { layout: false });
});

app.listen(3000);

I had to add @ line 16: var articleProvider= new ArticleProvider();