RxJava1: How to convert Observable<List<Observable<X>>> to Observable<List<X>>

Question

Can someone help me find the proper solution for this problem I face?

1. I have a backend service that give me Observables of the data I need, which are Events.
2. Form the Event I can get an EventGroup, which contains Ids of all events in the same group.
3. Next I can get all the Events that are part of this group.

However, I get a Observable<List<Observable<Event>>>, where I'd like to get a Observable<List<Event>>. How can I achive this, without actually subscribing to the nested Observables?

val events : Observable<List<Observable<Event>>> = 
   eventProvider.observable
      .flatMap { myBackend.getEventGroup(it.eventGroupId) }
      .map { 
         it.eventIds.map { myBackend.getEvent(it) } 
      }

TL:DR

How do I get Observable<List<X>> from a Observable<List<Observable<X>>>?

Answer 1

I figured it out, inspired by the answer of @akarnokd. @akarnokd answer expects all sources to be finite, but mine are all infinite. However I can live with partially infinite as well.

I need my resulting Observable to be infinite, because I want to receive a new list of Events whenever the eventProvider.observable emits something. However, I'm ok with the events I get back via myBackend.getEvent(it) to behave as a finite source.

val events : Observable<List<Event>> = 
   eventProvider.observable
      .flatMap { myBackend.getEventGroup(it.eventGroupId) }
      .map { it.eventIds.map { myBackend.getEvent(it) } }
      .map { it.toObservable().flatMap { it.first() }.toList() }
      .flatMap { it }