private function _getTitlesMap() {
        if (! is_array($this->_titlesMap)) {
            $map    = $this->_getMap();
            $result = array();

            foreach ($map as $key => $row) {

                // Title does not have to be unique. So if a title is used
                // twice we only use it for the first result.
                if (isset($row['code']) && $row['code'] && (! isset($result[$row['code']]))) {
                    $result[$row['code']] = $key;
                }
            }

            $this->_titlesMap = $result;
        }

        return $this->_titlesMap;
    }