FlatMap.java

getUserObservable()
    .flatMap(new Function<ApiUser, ObservableSource<UserDetail>>() { 
        @Override
        public ObservableSource<UserDetail> apply(ApiUser apiUser) throws Exception {
            return getUserDetailObservable(apiUser);
        }
    })
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(getObserver());