protected function sortOrderString()
    {
        $sortOrderComponents = [];
        foreach ($this->getSortOrder() as $sortCriterion) {
            $sortOrderComponents[] = $sortCriterion['fieldName'] . ':'
                    . (($sortCriterion['direction'] === 'descending') ? '0' : '1');
        }
        $sortOrderString = implode(',', $sortOrderComponents);

        return $sortOrderString;
    }