: TemplateTranformer. Otherwise, if you really must do it in Solr you can write : your own custom UpdateProcessor and plug it in:
Can't the SignatureUpdateProcessorFactory handle this using something like Lookup3Signature? (we should really offer a simple "ConcatSignature" implementation for generating predictable combination keys) http://wiki.apache.org/solr/Deduplication#solrconfig.xml -Hoss
