RxJava- How to backpressure a flatmap()

Question

Perhaps I'm overlooking a simple combination of operators (or an inherent cancellation behavior of RxJava altogether). But suppose I have a hot observable selectedItem that flatmaps to an RxJava-JDBC query.

@Test
public void testFlatMapBackPressure() { 
    Database db = null; //assign db

    BehaviorSubject<Integer> selectedItem = BehaviorSubject.create();

    //can I backpressure the queries so only the latest one is running, and any previous is cancelled?
    Observable<List<Integer>> currentValues = selectedItem.flatMap(i ->  db.select("SELECT VALUE FROM MY_TABLE WHERE ID =?")
            .parameter(i)
            .getAs(Integer.class)
            .toList());
}

How can I backpressure the flatMap() operator so it will ALWAYS only execute the latest query (and cancel any previous). I kind of want a backpressured flatMap operator to do something like this, where the "X" indicates a cancellation of the previous query

Is there a way to accomplish this? Or can it be accomplished already and I just do not see it?

Answer 1

Sounds like you need switchMap() instead of flatMap().

Returns a new Observable by applying a function that you supply to each item emitted by the source Observable that returns an Observable, and then emitting the items emitted by the most recently emitted of these Observables.