Create a filtered observable from 2 observable of list of items in RxJava

Question

I am working on a feature where I need to filter out the network response data based on local database data.

For example, my network layers return me a list of items and my database layer returns an observable list of ids. Now I want to only return those objects from network layer whose id matches anyone from the database layer response.

Below code fetches data from the network and saves the result to a database(cache).

factory.getRemoteDataStore()
                .searchForVenues(query)
                .toObservable()
                .distinctUntilChanged()
                .flatMap { venues ->
                    factory.getCacheDataStore()
                            .saveVenues(venues)
                            .andThen(Observable.just(venues))
                }

I also have a method that returns a list of venues that needs to be filtered

factory.getCacheDataStore().getDislikedVenues()

Now, how do I extend the previous chain to use getDislikedVenues() Observable to filter them from the response of network response?

Sorry for such noob question, I really am struggling with this.

Answer 1

One way of doing this is

factory.getCacheDataStore().getDislikedVenues()
    .flatMap { dislikedItems -> 
        factory.getRemoteDataStore()
            .searchForVenues(query)
            .toObservable()
            .distinctUntilChanged()
            .flatMapIterable { it }
            .filter { !dislikedItems.contains(it.id) }
            .toList()
            .toObservable()
    }
    .flatMap { venues ->
        factory.getCacheDataStore()
            .saveVenues(venues)
            .andThen(Observable.just(venues))
    }

P.S.: As I understand, factory.getRemoteDataStore().searchForVenues(query) returns Single or Maybe. In that case distinctUntilChanged() don't work because it relies on multiple emission (onNext()), but Single or Maybe can emit only one result.