protected function getFilterFieldValue(): string
    {
        if (isset($_SESSION[SlimPostgres::SESSION_KEY_ADMIN_LIST_VIEW_FILTER][$this->getFilterKey()][self::SESSION_FILTER_VALUE_KEY])) {
            return $_SESSION[SlimPostgres::SESSION_KEY_ADMIN_LIST_VIEW_FILTER][$this->getFilterKey()][self::SESSION_FILTER_VALUE_KEY];
        } else {
            return '';
        }
    }