private function getDocumentSource($field = null)
    {
        $docSource = $this->document->getSource() ? : [];
        $result    = $docSource;

        if (null !== $field) {
            $result = null;
            if (isset($docSource[$field])) {
                $result = is_array($docSource[$field]) ? current($docSource[$field]) : $docSource[$field];
            }
        }

        return $result;
    }