mongo-scala-driver: 4.3.1

Operations returning Observable[Void] cannot used map, flatMap, filter, etc., since they rely on onNext being called and Observable[Void] does not call onNext.

Two use cases where this is needed:

1. Returning a custom return type for operations that return Observable[Void]. For example:

rawDriverCollection.dropIndex(name).map(_ => CustomVoidMongoResult(request, someMetaData))

The .map(_ => CustomVoidMongoResult(request, someMetaData)) does not get executed, since MapObservable relies on onNext get called, but it never is called.

2. Chaining operations returning observables together using flatMap, where one of the (non-last) operations returns Observable[Void]. Fo example:

val resultObservable: Observable[String] = for {
    createdIndex <- createIndex(index)
     _ <- dropIndex(createdIndex)
     createdAgain <- createIndex(index)
   } yield createdAgain

This example uses for-comprehension syntactic sugar, where the arrows are maps and the yield is a flatMap. The last line createIndex(index) line never gets reached, since again MapObservable depends on onNext getting called.

One solution would be to change ToSingleObservableVoid to ToSingleObservableUnit, like:

  implicit class ToSingleObservableUnit(pub: => Publisher[Void]) extends SingleObservable[Unit] {
    val publisher = pub
    override def subscribe(observer: Observer[_ >: Unit]): Unit = SingleItemObservable(()).subscribe(observer)
  }

Unit is the Scala equivalent of Java Void, except that it can be instantiated.