Add value to a Mono<Entity> with Spring WebFlux

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I need to copy the property Friends (which is an ArrayList) from a Mono<PersonEntity> to a Mono<UserEntity> (which has not Friends property in the database), but I don't find the correct way to do it, so when i map the Mono<UserEntity> to Dto, the field Friends result to be an empty array [].

public Mono<Dto> findEntityByIdAndLabel(Long id, String label) {
    return getPersonByIdAndLabel(id, label).flatMap(person -> {
         return UserRepository.findByID(id);     
    })
            .switchIfEmpty(Mono.error(new EntityNotFoundException(entity.toString(), id.toString(), label)))
            .map(this::mapper);
}

I think I should add something after findById(id) but everything i tried until now didn't work. Thanks for your time

spring-webflux mapping spring java
1 Answers

Since you don't show PersonEntity and UserEntity we can only guess their properties and getter and setter methods. Still, something along the following lines should work:

public Mono<Dto> findEntityByIdAndLabel(Long id, String label) {
    return getPersonByIdAndLabel(id, label)
        .zipWith(UserRepository.findByID(id))
        .map(tuple -> {
            List friends = tuple.getT1().getFriends();
            UserEntity user = tuple.getT2():
            user.setFriends(friends);
            return user;
        })
        .switchIfEmpty(Mono.error(new EntityNotFoundException(entity.toString(), id.toString(), label)))
        .map(this::mapper);
}

The important thing is zipWith, which combines the result from a Mono and another Mono into a Tuple2 that you can then easily map. You can read more about this in the reference documentation.

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