public function getRatingValue()
    {
        if ($this->_ratingValue === null) {
            return null;
        }

        $range = new RangeValue($this->_ratingValue);
        $range->ensureRange($this->_worstRating ?: 0, $this->_bestRating ?: 5);

        return $this->_ratingValue = $range->getValue();
    }