protected function _initWeight(Zend_Search_Lucene_Interface $reader)
    {
        // Check, that it's a top-level query and query weight is not initialized yet.
        if ($this->_weight !== null) {
            return $this->_weight;
        }

        $this->createWeight($reader);
        $sum = $this->_weight->sumOfSquaredWeights();
        $queryNorm = $reader->getSimilarity()->queryNorm($sum);
        $this->_weight->normalize($queryNorm);
    }