public function getRequiredMessage($field)
    {
        if (!isset($this->_fields[$field])) {
            return null;
        }

        $defaultMessage = 'This field is required';
        if ($this->_useI18n) {
            $defaultMessage = __d('cake', 'This field is required');
        }

        return isset($this->_presenceMessages[$field])
            ? $this->_presenceMessages[$field]
            : $defaultMessage;
    }