RxJava subscribe on combination of flowables depending on each other

Question

Hey i am new to RxJava and try want to achieve the following: I have the following methods:

Flowable<List<Group>> getGroups();

Flowable<List<User>> getMembersForGroup(String groupid);

Now actually i want to get a result looking like:

Group g1 -> List members1

Group g2-> List members2 ....

Speaking: knowing the members of a group for every group.

I tried it with something like

getGroups().flatMap( //map getMembers(gid) for each gid  ) 

but i get stuck at the point "for each gid", because i dont know excactly what i actually want to return from flatMaps Func1 and how to handle it in flatMaps Func2.

Can somebody help?

Unfortunately i still have no idea, im not that much into the thinking of rxjava. Here is what i have got so far:

model.getGroupData()
        .flatMap(new Func1<QueryDocumentSnapshot,Flowable<Maybe<List<UserSchema>>>>() {
                     @Override
                     public Flowable<Maybe<List<UserSchema>>> call(QueryDocumentSnapshot doc) {
                         Groups groups = doc.toObject(Groups.class);
                             List<Maybe<List<UserSchema>>> memberflow =  new ArrayList<>();
                             for (GroupSchema g: groups){
                                 memberflow.add(model.getMembersOfGroup(g.getId()));

                             }
                             return Flowable.fromIterable(memberflow); 
                     }
                },
                 new Func2<QueryDocumentSnapshot, Flowable<Maybe<List<UserSchema>>>,Flowable>() {
                     @Override
                     public Flowable call(QueryDocumentSnapshot queryDocumentSnapshot, Flowable<Maybe<List<UserSchema>>> flowable) {
                    //zip?
                }
            });

As you can see i actually have a QueryDocumentSnapshot which does not really matter, cause it can be easily converted to the mentioned list.

So i am still not sure what i shall return from Func1, and hence what to zip.

Answer 1

This is one approach you could use (there are others, including the zip one):

groupApi
                .getGroups()
                .flatMap(Flowable::fromIterable)
                .map(Group::getId)
                .distinct()
                .flatMap(groupId -> groupApi.getMembersForGroup(groupId).map(users -> new Pair(groupId, users)))
                .subscribe(System.out::println);