title: Rails 3 - Ancestry gem method arrange not working
body: <hr>
  
  <p>hi people</p>
  
  <p>I've follow this tutorial: <a href="http://railscasts.com/episodes/262-trees-with-ancestry" rel="nofollow">http://railscasts.com/episodes/262-trees-with-ancestry</a> </p>
  
  <p>when I tried to make the "arrange (:order => :id)" I got this message</p>
  
  <pre><code>    undefined method `arrange' for #&lt;Array:0x8c19780&gt;
  
  Extracted source (around line #4):
  
  1: 
  2: &lt;h1&gt;Listing categories&lt;/h1&gt;
  3: 
  4: &lt;%= nested_categories @categories.arrange(:order =&gt; :id) %&gt;
  5: 
  6: &lt;br /&gt;
  7:
  </code></pre>
  
  <p>Can you help me, please?</p>
  
  <p>Thanks! </p>