如何使用 Lucene 的 DistinctValuesCollector？

Question

My objective is to collect distinct values of select fields to provided them as filter options for the frontend. DistinctValuesCollector seems to be the tool for this, however since I haven't found code sample and documentation except for the Javadocs I can't currently correctly construct this collector. Can anyone provide an example?

This is my attempt which doesn't deliver the desired distinct values of the field PROJEKTSTATUS.name .

val groupSelector = TermGroupSelector(PROJEKTSTATUS.name)
val searchGroup = SearchGroup<BytesRef>()
val valueSelector = TermGroupSelector(PROJEKTSTATUS.name)
val groups = mutableListOf(searchGroup)
val distinctValuesCollector = DistinctValuesCollector(groupSelector, groups, valueSelector)

That field is indexed as follows:

document.add(TextField(PROJEKTSTATUS.name, aggregat.projektstatus, YES))
document.add(SortedDocValuesField(PROJEKTSTATUS.name, BytesRef(aggregat.projektstatus)))

Answer 1

Thanks to @andrewJames's hint to a test class I could figure it out:

fun IndexSearcher.collectFilterOptions(query: Query, field: String, topNGroups: Int = 128, mapper: Function<String?, String?> = Function { it }): Set<String?> {
    val firstPassGroupingCollector = FirstPassGroupingCollector(TermGroupSelector(field), Sort(), topNGroups)
    search(query, firstPassGroupingCollector)
    val topGroups = firstPassGroupingCollector.getTopGroups(0)
    val groupSelector = firstPassGroupingCollector.groupSelector
    val distinctValuesCollector = DistinctValuesCollector(groupSelector, topGroups, groupSelector)
    search(query, distinctValuesCollector)
    return distinctValuesCollector.groups.map { mapper.apply(it.groupValue.utf8ToString()) }.toSet()
}