Ember.js: Grouping By Date (promises)

App.LineItemsController = Ember.ArrayController.extend({
  groupedContent: (function() {
    var lineItems = this.get('content');
    
    if(!lineItems.get('isLoaded')) {
      return null;
    }
    
    return lineItems.reduce(function(grouped, object, index, content) {
      if(content.objectAt(index-1) &&
        object.get('date').valueOf() == content.objectAt(index-1).get('date').valueOf()) {
          grouped.get('lastObject').pushObject(object);
      }
      else {
        grouped.pushObject(Em.A([object]));
      }
      return grouped;
    }, Em.A([]));
  }).property('content.isLoaded')
});