How to use db references with reactive Spring Data MongoDB?

TL;DR

There's no DBRef support in reactive Spring Data MongoDB and I'm not sure there will be.

Explanation

Spring Data projects are organized into Template API, Converter and Mapping Metadata components. The imperative (blocking) implementation of the Template API uses an imperative approach to fetch Documents and convert these into domain objects. MappingMongoConverter in particular handles all the conversion and DBRef resolution. This API works in a synchronous/imperative API and is used for both Template API implementations (imperative and the reactive one).

Reuse of MappingMongoConverter was the logical decision while adding reactive support as we don't have a need to duplicate code. The only limitation is DBRef resolution that does not fit the reactive execution model.

To support reactive DBRefs, the converter needs to be split up into several bits and the whole association handling requires an overhaul.

Reference : https://jira.spring.io/browse/DATAMONGO-2146

Recommendation

Keep references as keys/Id's in your domain model and look up these as needed. zipWith and flatMap are the appropriate operators, depending on what you want to archive (enhance model with references, lookup references only).

On a related note: Reactive Spring Data MongoDB comes partially with a reduced feature set. Contextual SpEL extension is a feature that is not supported as these components assume an imperative programming model and thus synchronous execution.

For the first point, I finally achieve doing what I wanted :

public Mono<User> getUser(String login) {
   return userRepository.findByLogin(login)
         .flatMap( user ->
              Mono.just(user)
              .zipWith(profileRepository.findAllById(user.getProfileObjectIds())
                  .collectionList(),
                  (u, p) -> {
                       u.setProfiles(p);
                       return u;
                   })
            );
}