protected function _addSlugFromMultiple(Garp_Model_Db $model, array &$targetData, array $referenceData) {
        $baseFields = $this->_config['baseField'];
        $slugField  = $this->_config['slugField'][0];
        $baseData = array();
        if (!empty($targetData[$slugField])) {
            return;
        }
        $lang = null;
        if (isset($referenceData[Garp_Model_Behavior_Translatable::LANG_COLUMN])) {
            $lang = $referenceData[Garp_Model_Behavior_Translatable::LANG_COLUMN];
        }
        foreach ((array)$baseFields as $baseColumn) {
            $baseData[] = $this->_getBaseString($baseColumn, $referenceData);
        }
        $baseData = implode(' ', $baseData);
        $targetData[$slugField] = $this->generateUniqueSlug($baseData, $model, $slugField, $lang);
    }