How to sort an ArrayController backed by Ember-Model?

Prior to using Ember-Model sorting my ArrayController with sortProperties and sortAscending worked as expected. Lately I switched to Ember-Model and the sorting feature doesn't work anymore.

Code so far:

App.Movie = Ember.Model.extend({
  rating: Ember.attr(),
  title: Ember.attr(),
  watched: Ember.attr(),
  year: Ember.attr(),
});

App.MoviesIndexController = Ember.ArrayController.extend({
  sortProperties: null,
  sortAscending: null,

  actions: {
    sortByAttribute: function (attr) {
      if (this.get('sortProperties')) {
        if (this.get('sortProperties')[0] === attr) {
          this.toggleProperty('sortAscending');
        } else {
          this.set('sortProperties', [attr]);
        }
      } else {
        this.set('sortProperties', [attr]);
        this.set('sortAscending', true);
      }
    }
  }
});

<table>
  <thead>
    <tr>
      <th {{action "sortByAttribute" "year"}}>Year </th>
    </tr>
  </thead>
  ...
</table>

Any idea?

Thx