Is it a bad idea to subscribe another observable inside doOnNext?

[Raymond Chan]

I'm a bit new to RxJava and currently I'm migrating some old code to reactive approach.
Here's the situation, I would like to display a list of posts from local or remote

Observable<List<Post>> local = createLocalObservable();
Observable<List<Post>> remote = createRemoteObservable();
Observable<List<Post>> combined = Observable.concat(local, remote)
                                .doOnNext(o -> {
                                    //... some logging etc...
                                });

In the old logic, I have an extra API call inside the remote API call returned and run in a separate thread.

So my question is, how can I make the extra API call with single Observable?

What I came up so far:

createRemoteObservable()
.doOnNext(posts -> {
    extraApiCallObservable(posts)
    .subscribeOn(Schedulers.io())
    .subscribe();
});

However, that doesn't look nice and moreover, I may need to update the UI according to the extra API call's result.
How can I achieve parallelism for this case?

[Łukasz Marczak]

Maybe zip operator will help you with parallel execution:
Observable.zip(o1(),o2(), new Func2<...>)
.map(..) and so on.

[George Campbell]

What about flatMap.

        createRemoteObservable().flatMap(posts -> {

            return extraApiCallObservable(posts).subscribeOn(Schedulers.io()).toCompletable().andThen(Observable.just(posts));

        });