How can I use custom loopback models in server.js

I'd like to use custom loopback model in server.js like below.

server/server.js

app.start = function() {
  // start the web server
  return app.listen(function() {
    app.emit('started');
    console.log('Web server listening at: %s', app.get('url'));
  });
};

if (require.main === module) {
  var server = app.start();
  app.models.Foo.something(); // <- call method here
}

common/models/foo.js

module.exports = function(Foo) {
  Foo.prototype.something = function() {
    console.log('Hi');
    return true;
  };

  Foo.setup = function() {
    Foo.base.setup.apply(this, arguments);
  };
  Foo.setup();
};

ref: https://gist.github.com/bajtos/213d5dae87e19f47db5d

But actually I got a error like this.

/Projects/repo/server/server.js:45   
  app.models.Foo.something();
                     ^
TypeError: Object function ModelConstructor(data, options) {
      if (!(this instanceof ModelConstructor)) {
        return new ModelConstructor(data, options);
      }
      if (ModelClass.settings.unresolved) {
        throw new Error('Model ' + ModelClass.modelName + ' is not defined.');
      }
      ModelBaseClass.apply(this, arguments);
    } has no method 'something'
    at Object.<anonymous> (/Projects/repo/server/server.js:45:22)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)

The method seems not to be defined. How to use custom methods in launching server and how can I correct it?