I finally solved the issue creating a custom Update Request Processor, as explained here: http://wiki.apache.org/solr/UpdateRequestProcessor
My processor is as follows:
package com.mycompany.solr;
import java.io.IOException;
import org.apache.solr.common.SolrInputDocument;
import org.apache.solr.request.SolrQueryRequest;
import org.apache.solr.response.SolrQueryResponse;
import org.apache.solr.update.AddUpdateCommand;
import org.apache.solr.update.processor.UpdateRequestProcessor;
import org.apache.solr.update.processor.UpdateRequestProcessorFactory;
public class LastModifiedMergeProcessorFactory
extends UpdateRequestProcessorFactory {
@Override
public UpdateRequestProcessor getInstance(SolrQueryRequest req,
SolrQueryResponse rsp, UpdateRequestProcessor next) {
return new LastModifiedMergeProcessor(next);
}
}
class LastModifiedMergeProcessor extends UpdateRequestProcessor {
public LastModifiedMergeProcessor(UpdateRequestProcessor next) {
super(next);
}
@Override
public void processAdd(AddUpdateCommand cmd) throws IOException {
SolrInputDocument doc = cmd.getSolrInputDocument();
Object metaDate = doc.getFieldValue( "last_modified" );
Object fileDate = doc.getFieldValue( "file_date" );
if( metaDate == null && fileDate != null) {
doc.addField( "last_modified", fileDate );
}
// pass it up the chain
super.processAdd(cmd);
}
}
Where file_date is a field I set with the file modification date at import time.