[TYPO3-Solr] How to index only certain news category

[Jonas Götze]

Hi List,

in the wiki [1] is a note about a usecase where someone needs to index 
tt_news items of a certain category. How should I filter for a certain 
category in this example?
I guess I should use addtionalWhereClause, but how can I access the 
needed mm-Table for the categories?

Thanks for your help!
Regards
Jonas

[1] 
http://forge.typo3.org/projects/extension-solr/wiki/Tx_solrindex#queueindexingConfigurationtable