MarksSolution

  public record RecordA(String id) { }
  public record RecordB(String id, String name) { }
  @Test
  public void testFilteringRecordAListWithIdsNotFoundOnRecordBList() {

    final Mono<List<RecordA>> recordAMonoList = Mono.just(
        List.of(new RecordA("1"), new RecordA("2"), new RecordA("3"), new RecordA("4")));
    final Mono<List<RecordB>> recordBMonoList = Mono.just(
        List.of(new RecordB("1", "Record 1"), new RecordB("2", "Record 2")));

    recordAMonoList.zipWith(recordBMonoList).map(this::compareAndFilterLists).subscribe(
        System.out::println, System.err::println, () -> System.out.println("Finished filtering RecordA list")
    );
  }

  private List<RecordA> compareAndFilterLists(Tuple2<List<RecordA>, List<RecordB>> tuple) {
    return tuple.getT1()
        .stream()
        .filter(r -> tuple.getT2().stream().noneMatch(b -> b.id().equals(r.id())))
        .collect(
            Collectors.toList());
  }