title: CakePHP Tree Behavior issue - first entry
body: <p>I'm having an issue with initiating the Tree behavior issue in CakePHP. I have set up the table with <code>lft</code>, <code>rght</code> and <code>parent_id</code> columns.</p>
  
  <p>I'm adding a first entry which other can refer to with the <code>parent_id</code> column. However, the referring does not work as I want.</p>
  
  <p>The first entry is added to the database as expected. If I don't specify <code>parent_id</code> that column will get <code>NULL</code> as database value. The <code>lft</code> and <code>rght</code> columns is also getting <code>NULL</code> as value. I'm not sure if that's what it's supposed to do.</p>
  
  <p>When I'm trying to add a new entry after the initial one, the save will fail.</p>
  
  <p>I've tried to set the <code>lft</code> and <code>rght</code> columns to <code>1, 1</code> and <code>1, 2</code> on the first entry, but without any luck.</p>
  
  <p>Am I doing something wrong here?</p>