Reactor - how to collect found and not found entities into a map, with id as key?

Imagine that I upload a list of user ids into server to find them; I want to know which are the id not found in database, so I will be aware of that my local cache of db data, or my real data is broken. Missing ids are useful as feedback in UI and in logs.

I want to construct a map with key as input id, and value as found entity user in cache/database. If not found, the value is null. But, ReactiveCrudRepository will return Mono.empty() if findById(id) returns nothing, and consequent mapping will not include not found entity. In test I get error:

Expected: is <true>
     but: was <false>
java.lang.AssertionError: 
Expected: is <true>
     but: was <false>

I am doing:

return Flux.fromIterable(ids)
            .concatMap(repository::findById) // <--------------- this step only gets found entities
            .collectMap(Customer::getId); // <------------------ so here I get only existent elements(in test, a map with 2 elements)

The key is to know what id is found and what is not. How can I do this in another way in Reactor 3?

The test:

    @Test
    void findAllByIdAndMissing() {
        // given
        Customer customer1 = Customer.builder().id("id1").build();
        Customer customer2 = Customer.builder().id("id2").build();
        // given mocked
        when(cache.getAllPresent(ArgumentMatchers.any())).thenReturn(Collections.emptyMap());
        when(repository.findById("id1")).thenReturn(Mono.just(customer1));
        when(repository.findById("id2")).thenReturn(Mono.just(customer2));
        when(repository.findById("id3")).thenReturn(Mono.empty());

        // when
        Map<String, Customer> actual = service.findAllByIdAndMissing(Set.of("id3", "id2", "id1")).block();

        // then
        assertThat(actual.keySet().contains("id1"), is(true));
        assertThat(actual.keySet().contains("id2"), is(true));
        assertThat(actual.keySet().contains("id3"), is(true));
    }

If I add log():

:08:03.072 [Test worker] INFO reactor.Flux.Iterable.1 - | onSubscribe([Synchronous Fuseable] FluxIterable.IterableSubscription)
13:08:03.078 [Test worker] INFO reactor.Flux.FlatMap.2 - onSubscribe(FluxFlatMap.FlatMapMain)
13:08:03.080 [Test worker] INFO reactor.Flux.FlatMap.2 - request(unbounded)
13:08:03.081 [Test worker] INFO reactor.Flux.Iterable.1 - | request(256)
13:08:03.082 [Test worker] INFO reactor.Flux.Iterable.1 - | onNext(id1)
13:08:03.084 [Test worker] INFO reactor.Flux.FlatMap.2 - onNext(Customer(super=Auditable(super=Customer@196b1, createdAt=null, createdBy=null, modifiedAt=null, modifiedBy=null), schemaVersion=1, internalVersion=null, id=id1))
13:08:03.145 [Test worker] INFO reactor.Flux.Iterable.1 - | onNext(id2)
13:08:03.146 [Test worker] INFO reactor.Flux.FlatMap.2 - onNext(Customer(super=Auditable(super=Customer@196b2, createdAt=null, createdBy=null, modifiedAt=null, modifiedBy=null), schemaVersion=1, internalVersion=null, id=id2))
13:08:03.147 [Test worker] INFO reactor.Flux.Iterable.1 - | onNext(id3)
13:08:03.148 [Test worker] INFO reactor.Flux.Iterable.1 - | onComplete()
13:08:03.149 [Test worker] INFO reactor.Flux.FlatMap.2 - onComplete()

Not sure if there is a more idiomatic way to do this but you can easily achieve this with a helper class:

Flux.fromIterable(ids)
    .concatMap(id -> repository.findById(id).map(Result::found).defaultIfEmpty(Result.notFound(id)))
    .collectMap(Result::getId, Result::getCustomer);


@Data
@AllArgsConstructor(access = AccessLevel.PRIVATE)
class Result {
    private final String id;
    private final Customer customer;

    public static Result found(Customer customer) {
        return new Result(customer.getId(), customer);
    }

    public static Result notFound(String id) {
        return new Result(id, null);
    }
}