You can create a custom update processor. The passed AddUpdateCommand object has an accessor to the SolrInputDocument you're about to add. In the processAdd method you can add a new field with whatever you want.
The wiki has a good example: http://wiki.apache.org/solr/UpdateRequestProcessor > Hello, > > I'm trying to add a field that counts the number of terms in a document to > my schema. So far I've been computing this value at query-time. Is there > how I could compute this once only and store the field? > > final SolrIndexSearcher searcher = request.getSearcher(); > final SolrIndexReader reader = searcher.getReader(); > final String content = "content"; > > final byte[] norms = reader.norms(content); > final int[] docLengths; > if (norms == null) { > docLengths = null; > } else { > docLengths = new int[norms.length]; > int i = 0; > for (byte b : norms) { > > float docNorm = > searcher.getSimilarity().decodeNormValue(b); int docLength = 0; > if (docNorm != 0) { > docLength = (int) (1 / docNorm); //reciprocal > } > docLengths[i++] = docLength; > } > ... > final NumericField docLenNormField = new > NumericField(TestQueryResponseWriter.DOC_LENGHT); > docLenNormField.setIntValue(docLengths[id]); > doc.add(docLenNormField);
