title: how does TreeBehavior works in cakephp 3
body: <p>I'm following this <a href="http://book.cakephp.org/3.0/en/orm/behaviors/tree.html" rel="nofollow">http://book.cakephp.org/3.0/en/orm/behaviors/tree.html</a>
  to make simple category tree.</p>
  
  <p>You can make a node a root in the tree by setting the ‘parent_id’ column to null:</p>
  
  <pre><code>$aCategory = $categoriesTable-&gt;get(10);
  $aCategory-&gt;parent_id = null;
  $categoriesTable-&gt;save($aCategory);
  </code></pre>
  
  <p>this does not work to add new root node (I didn't understand why it's 10 there)
  I'm have empty table</p>
  
  <p>please guide, me how to get started with root and the add child nodes to it</p>
  
  <pre><code>CREATE TABLE IF NOT EXISTS `categories` (
    `id` int(11) NOT NULL,
    `parent_id` int(11) DEFAULT NULL,
    `category_slug` varchar(250) DEFAULT NULL,
    `category_name` varchar(250) DEFAULT NULL,
    `lft` int(11) DEFAULT NULL,
    `rght` int(11) DEFAULT NULL
  ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
  </code></pre>