public function isChildOf($user_id, $fk) {
        if (!$user_id || !$fk) {
            return false;
        }

        try {
            $aros = $this->node(array(
                'model' => USER_MODEL,
                'foreign_key' => $user_id
            ));
        } catch (Exception $e) {
            return false;
        }

        if ($aros) {
            foreach ($aros as $aro) {
                if (is_string($fk) && $aro['RequestObject']['alias'] == $fk) {
                    return true;
                } else if (is_numeric($fk) && $aro['RequestObject']['id'] == $fk) {
                    return true;
                }
            }
        }

        return false;
    }