CouchDB with lucene get uniq tags

[faust 1111]

I have
Contents
   tags

i try implement full text search in tags for autocompleter
i it possible with lucene?
i need get list of uniq tags

now i try

function(doc) {
    if(doc.tags) {
      var ret = new Document();

      doc.tags.forEach(function(tag) {
        ret.add(tag, {"store": "yes"});
      });

      return ret;
}

but its not work for me.
its return me all docs + all tags related, not only i search*.