Question

I have a Lucene index that contains documents that have a “type” field, this field can be one of three values “article”, “forum” or “blog”. I want the user to be able to search within these types (there is a checkbox for each document type)

How do I create a Lucene query dependent on which types the user has selected?

A couple of prerequisites are:

• If the user doesn’t select one of the types, I want no results from that type.
• The ordering of the results should not be affected by restricting the type field.

For reference if I were to write this in SQL (for a “blog or forum search”) I’d write:

SELECT * FROM Docs
WHERE [type] in ('blog', 'forum')

Answer 1

For reference, should anyone else come across this problem, here is my solution:

IList<string> ALL_TYPES = new[] { "article", "blog", "forum" };
string q = ...; // The user's search string
IList<string> includeTypes = ...; // List of types to include
Query searchQuery = parser.Parse(q);
Query parentQuery = new BooleanQuery();
parentQuery.Add(searchQuery, BooleanClause.Occur.SHOULD);
// Invert the logic, exclude the other types
foreach (var type in ALL_TYPES.Except(includeTypes))
{
    query.Add(
        new TermQuery(new Term("type", type)),
        BooleanClause.Occur.MUST_NOT
    );
}
searchQuery = parentQuery;

I inverted the logic (i.e. excluded the types the user had not selected), because if you don’t the ordering of the results is lost. I’m not sure why though…! It is a shame as it makes the code less clear / maintainable, but at least it works!