saving users with ACL in cakephp

gistfile1.php

<?php
    /**
     * Cambia el parent_id del aro asociado al correspondiente al nuevo grupo
     */
    function afterSave($created) {
        if(!$created && $this->id && isset($this->data['Usuario']['grupo_id'])) {
            $aro = $this->Aro->find('first',
                array(
                    'conditions' => array(
                        'model' => 'Usuario',
                        'foreign_key' => $this->id
                    )
                )
            );
            $aro_parent = $this->Aro->find('first',
                array(
                    'conditions' => array(
                        'model' => 'Grupo',
                        'foreign_key' => $this->data['Usuario']['grupo_id']
                    ),
                    'fields' => array('Aro.id')
                )
            );
            $aro['Aro']['parent_id'] = $aro_parent['Aro']['id'];
            $this->Aro->save($aro);
        }
    }
?>