Question

Without using the latest version of Lucene with faceting enabled, I am trying to implement multi-facets hitcounts. I have found a great post to get started, but not sure of the next step of multi-value facets.

The below example shows code for a singular example, e.g. Category field, and getting count for each one.

private static void FacetedSearch(string indexPath, string genre, string term)
{
    // create searcher
    var searcher = new IndexSearcher(indexPath);

    // first get the BitArray result from the genre query
    var genreQuery = new TermQuery(new Term("genre", genre));
    var genreQueryFilter = new QueryFilter(genreQuery);
    BitArray genreBitArray = genreQueryFilter.Bits(searcher.GetIndexReader());
    Console.WriteLine("There are " + GetCardinality(genreBitArray) + " document with the genre " + genre);

    // Next perform a regular search and get its BitArray result
    Query searchQuery = MultiFieldQueryParser.Parse(term, new[] {"title", "description"}, 
                                        new[] {BooleanClause.Occur.SHOULD, BooleanClause.Occur.SHOULD}, 
                                        new StandardAnalyzer());
    var searchQueryFilter = new QueryFilter(searchQuery);
    BitArray searchBitArray = searchQueryFilter.Bits(searcher.GetIndexReader());
    Console.WriteLine("There are " + GetCardinality(searchBitArray) + " document containing the term " + term);

    // Now do the faceted search magic, combine the two bit arrays using a binary AND operation
    BitArray combinedResults = searchBitArray.And(genreBitArray);
    Console.WriteLine("There are " + GetCardinality(combinedResults) + 
                " document containing the term " + term + " and which are in the genre " + genre);
}

But, if I have two separate fields, e.g. Category and Topic, so that I could have:

• Category1, Category2

• Topic1, Topic2

in my UI, if each of these is a checkbox, I could select both category1, and category2, then the counts for topic1, topic2 would be different than if I just select category1.

Not sure how to do the bitarray searches for that multiple instance.

Answer 1

To work out the facet counts you need to get the cardinality of two searches. In your example above your first search is for items with genre = genre, and then counting the facets for your term in title and description.

To achieve what you are after, you need to create your base search, which will be a query where the product is in both category1 AND category2, so to do this you would have a BooleanQuery to get your initial result set, this would be something like

var booleanQuery = new BooleanQuery();
booleanQuery.Add(new Term("category", "Category1"));
booleanQuery.Add(new Term("category", "Category2"));
var categoryFilter = new QueryFilter(booleanQuery);
var catBits = categoryFilter.Bits(searcher.GetIndexReader());

To then count for each facet you would create a query for each facet individually (in a loop) and then do your getcardinality call between your category query and each facet query.

foreach (var topic in myTopics) {
    var topicQueryFilter = new QueryFilter(new TermQuery(new Term("topic", topic)));
    var topicBits = topicQueryFilter.Bits(searcher.GetIndexReader());

    var facetResults = catBits.And(topicBits);
    var topicCount = GetCardinality(facetResults);
}