cakePHP Treeview Model

<?php
App::uses('AppModel', 'Model');
class Histories extends AppModel {
  public $useTable = 'histories';
  var $displayField = 'history_name';
  var $hasMany = array(
      'ChildPage' =>
      array('className' => 'Histories',
          'foreignKey' => 'parent_id'
      ),
  );

  public function __getAll() {

    $q = $this->find('all', array(
        'conditions' => array("Histories.parent_id " => NULL),
        'recursive' => 2,
        'order' => array('Histories.id' => 'asc')
            ));
    return $q;
  }
}