title: ember.js - extract distinct properties from array
body: <p>lets say I have the following array, and this is used in my controller</p>
  
  <pre><code>songs = [
    {trackNumber: 4, title: 'Ob-La-Di, Ob-La-Da', genre: 'pop'},
    {trackNumber: 2, title: 'Back in the U.S.S.R.', genre: 'rock'},
    {trackNumber: 3, title: 'Glass Onion', genre: 'pop'},
  ];
  </code></pre>
  
  <p>I would like to have a property on my controller that returns an array of unique genres</p>
  
  <p>eg</p>
  
  <pre><code>genres: function() {
    ...
  }...
  </code></pre>
  
  <p>and in this case would return</p>
  
  <pre><code>['pop', 'rock']
  </code></pre>
  
  <p>is there some elegant way to do this with ember, using computed properties an or observers?</p>