How to preserve order of list after applying flatMap() to it?

Question

The call to api.bookings returns a list of bookings in a predefined order. The call to api.order() returns a booking but with some extra info. After applying flatMap() the order of the bookings list gets messed up. (bookings.toList() is just a list if Bookings). How can I get the values in the zip function in the order as emitted by .from(bookings.toList()). Note: I can't use toSortedList as I have no way to compare two bookings.

bookingsSubscription = api.bookings()
                    .flatMap(bookings -> 
                        Observable.zip(
                            Observable 
                                .from(bookings.toList()) 
                                .flatMap(booking -> api.order(booking.orderId)),

                            Observable.from(bookings.toList()),

                            (newBooking, oldBooking) -> {
                               // at this point newBooking != oldBooking
                            }
                    ).toList())
                    .subscribe(callback);

api.order

Answer 1

concatMap will work but won't give the concurrency which you want. concatMapEager on the other hand will.

An equivalent option is to create a List<Observable> manually and use Observable.zip(Iterable, FuncN) to combine the list. This will have the same effect as concatMapEager.

See RxJava issue 3017 for a discussion.