public function filter_field($key, $value)
    {
        if ($key == 'emailMD5' && false !== strpos($value, '@')) {
            return md5(strtolower($value));
        }

        if (($key == 'usernameMD5' || $key == 'passwordMD5')
            && strlen($value) != 32
        ) {
            return md5(strtolower($value));
        }

        return $value;
    }