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load the query parser drop-in jar https://wiki.apache.org/solr/SolrPlugins#How_to_Load_Plugins
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enable the query parser by adding into solrconfig.xml
<queryParser name="scorejoin" class="join.ScoreJoinQParserPlugin"/>
- call it almost like standard join query parser http://wiki.apache.org/solr/Join but you also can specify 'score' parameter
q={!scorejoin from=prod_id to=id score=max}title:ipod
- available score parameter values are: None, Avg, Max, Total. Lowercased valued should also work, follow http://lucene.apache.org/core/4_6_0/join/org/apache/lucene/search/join/ScoreMode.html for the description.