protected function extractNodes(CNabuDataObject $item)
    {
        $main_index_name = $this->list->getIndexedFieldName();
        if (($item->isValueNumeric($main_index_name) || $item->isValueGUID($main_index_name)) &&
            ($item->isValueString($this->key_field) || $item->isValueNumeric($this->key_field))
        ) {
            $key = $item->getValue($this->key_field);
            $retval = array(
                'key' => $key,
                'pointer' => $item->getValue($main_index_name)
            );
            if ($item->isValueNumeric($this->order_field) || $item->isValueString($this->order_field)) {
                $retval['order'] = $item->getValue($this->order_field);
            }
            $retval = array($key => $retval);
        } else {
            $retval = null;
        }

        return $retval;
    }