public function getIndex($index)
    {
        if (is_string($index) &&
            is_array($this->secondary_indexes) &&
            array_key_exists($index, $this->secondary_indexes)
        ) {
            return $this->secondary_indexes[$index];
        } else {
            throw new ENabuCoreException(ENabuCoreException::ERROR_INVALID_INDEX, array(print_r($index, true)));
        }
    }