How to do a join for every object in a Flux with a database call returning a Mono

Question

I have two database calls one returning a Flux and another returning a Mono

The LegacyPerson class contains an legacyPersonId and a name. The PersonKey class contains and legacyPersonId and a PersonID

I want to get as result an Mono<List> where the Person class contains an legacyPersonId and an PersonId. So i have to determine the id for every legacyPerson in the flux without blocking.

        class LegacyPerson {
            Integer legacyId;
            String name;
        }

        @Builder
        class Person {
            UUID id;
            Integer legacyId;
            String name;
        }

        class PersonKey {
            UUID id;
            Integer legacyId;
        } 

two repositories:

        @Repository
        public interface RepositoryKey extends ReactiveCrudRepository<PersonKey, UUID> {
            Mono<PersonKey> findByPensioennummer(Integer pensioennummer);
        }
        @Repository
        public interface RepositoryPerson extends ReactiveCrudRepository<LegacyPerson, UUID> {
            Flux<LegacyPerson> findByName(String name);
        }

This is what i have tried

       class Service {
            RepositoryKey repKey;
            RepositoryPerson repPerson;
            public Mono<List<Person>> getPersons(String name) {
                Flux<LegacyPerson> legacyPersonFlux = repPerson.findByName(name);

                legacyPersonFlux.map(person -> map(repKey.findByPensioennummer(person.legacyId), person));
            }

            private Person map(UUID id, LegacyPerson legacyPerson) {
                return Person.builder().id(id).name(legacyPerson.name).legacyId(legacyPerson.legacyId).build();
            }
        }

I get an error that the map method expects an UUID an not an Mono. I can change it to Mono but I don't want to change the Id in my domain object Person.

Any help would be greatly appreciated!

Answer 1

That doesn't make sense. public Person getPerson(String name) is not reactive. You need, at least, public Mono<Person> getPerson(String name) . Further, what do you plan to do if the Flux<LegacyPerson> findByName(String name) returns more than one result? At first consider changing it to Mono<LegacyPerson> findByName(String name) .

RepositoryKey repKey = new RepositoryKey();;
RepositoryPerson repPerson = new RepositoryPerson();

public Mono<Person> getPerson(String name) {
    Flux<LegacyPerson> legacyPersonFlux = repPerson.findByName(name);
    
    return  legacyPersonFlux.next()
            .flatMap(legacyPerson -> repKey.findByPensioennummer(legacyPerson.legacyId).map(pk->map(pk.id, legacyPerson)));
}

private Person map(UUID id, LegacyPerson legacyPerson) {
    return Person.builder().id(id).name(legacyPerson.name).legacyId(legacyPerson.legacyId).build();
}   
private void run() {
    getPerson("me").subscribe(System.out::println);
}



static class LegacyPerson {
    Integer legacyId;
    String name;
}

@Builder
@Data
static class Person {
    UUID id;
    Integer legacyId;
    String name;
}

static class PersonKey {
    UUID id;
    Integer legacyId;
}
public class RepositoryKey {
    Mono<PersonKey> findByPensioennummer(Integer pensioennummer) {
        PersonKey pk = new PersonKey();
        pk.legacyId = pensioennummer;
        pk.id = UUID.randomUUID();
        return Mono.just(pk);
    }
}
public class RepositoryPerson {
    Flux<LegacyPerson> findByName(String name) {
        LegacyPerson lp = new LegacyPerson();
        lp.legacyId = 1;
        lp.name = name;
        return Flux.just(lp); 
    }
}