Lucene - How can you modify a Query object's terms? - lucene

Question

I want to add new fields to my Lucene-based search engine site, however I want to be able to intercept queries and modify them before I pass them on to the Searcher.
For example each document has the field userid so you can search for documents authored by a particular user by their ID, e.g. foo bar userid:123 however I want to add the ability to search by username.
I'd like to add a field user:RonaldMcDonald to queries (not to documents), however I want to be able to intercept that term and replace it with an equivalent userid:123 term (my own code would be responsible for converting "RonaldMcDonald" to "123").
Here's the simple code I'm using right now:
Int32 get = (pageIndex + 1) * pageSize;
Query query;
try {
query = _queryParser.Parse( queryText );
} catch(ParseException pex) {
log.Add("Could not parse query.");
throw new SearchException( "Could not parse query text.", pex );
}
log.Add("Parsed query.");
TopDocs result = _searcher.Search( query, get );
I've had a look at the Query class, but I can't see any way to retrieve, remove, or insert terms.

Answer 1

You can subclass the QueryParser and override NewTermQuery.
QP qp = new QP("user", new SimpleAnalyzer());
var s = qp.Parse("user:RonaldMcDonald data:[aaa TO bbb]");
Where s is will be userid:123 data:[aaa TO bbb]
public class QP : QueryParser
{
Dictionary<string, string> _dict =
new Dictionary<string, string>(new MyComparer()) {{"RonaldMcDonald","123"} };
public QP(string field, Analyzer analyzer) : base(field, analyzer)
{
}
protected override Query NewTermQuery(Term term)
{
if (term.Field() == "user")
{
//Do your username -> userid mapping
return new TermQuery(new Term("userid", _dict[term.Text()]));
}
return base.NewTermQuery(term);
}
//Case insensitive comparer
class MyComparer : IEqualityComparer<string>
{
public bool Equals(string x, string y)
{
return String.Compare(x, y, true, CultureInfo.InvariantCulture)==0;
}
public int GetHashCode(string obj)
{
return obj.ToLower(CultureInfo.InvariantCulture).GetHashCode();
}
}
}