How to Fetch objects, using references from other objects, in Reactive Spring Data?

1. I'm new to MongoDB and WebFlux. I'm trying to retrieve a 'Post' with its 'Comments'(List listComments) using 'CommentsIds'(List idComments) from this 'Post' POJO:

2. That's my POJO/Entity:

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@Document(collection = "post")
public class Post implements Serializable {

    private static final long serialVersionUID = -6281811500337260230L;

    @EqualsAndHashCode.Include
    @Id
    private String id;
    private Date date;
    private String title;
    private String body;
    private AuthorDto author;
    private Comment comment;
    private List<Comment> listComments = new ArrayList<>();
    private List<String> idComments = new ArrayList<>();
}

3. It is my Service (findPostByIdShowComments + findCommentsByPostId):

    public Mono<Post> findPostByIdShowComments(String id) {
        return postRepo
                .findById(id)
                .switchIfEmpty(postNotFoundException())
                .flatMap(postFound -> {

                    Flux<Comment> commentFlux = commentService
                            .findCommentsByPostId(postFound.getId());

                    Flux<Post> postFlux = commentFlux
                            .flatMap(comment -> {
                                postFound.setListComments(new ArrayList<>());
                                postFound.getListComments()
                                         .add(comment);
                                return Mono.just(postFound);
                            });

                    return Mono.just(postFound);
                });
    }


    public Flux<Comment> findCommentsByPostId(String id) {
        return postRepo
                .findById(id)
                .switchIfEmpty(postNotFoundException())
                .thenMany(commentRepo.findAll())
                .filter(comment1 -> comment1.getIdPost()
                                            .equals(id));

    }

4. That's my controller:

    @GetMapping(FIND_POST_BY_ID_SHOW_COMMENTS)
    @ResponseStatus(OK)
    public Mono<Post> findPostByIdShowComments(@PathVariable String id) {
        return postService.findPostByIdShowComments(id);
    }

5. As a result, I am not able to FEtch my 'Comments' and load them in the POJO, and consequently, I could not show them in the Rest Response Json. As a 'Json result' I am getting:

{
    "id": "5ff0ed93b3268a39dd3bd7b9",
    "date": "2011-11-11T00:00:00.000+00:00",
    "title": "Lake Cliffordside",
    "body": "North Sanfordborough",
    "author": {
        "id": "5ff0ed93b3268a39dd3bd7b6",
        "name": "melissa"
    },
    "comment": null,
    "listComments": [], //TROUBLE: As you can see, The CommentObjects are not coming
    "idComments": [
        "5ff0edb2b3268a39dd3bd7bc",
        "5ff0edbab3268a39dd3bd7bd"
    ]
}

Thanks a lot for your Guys help.

The reason why you don't receive comments is that your commentFlux and postFlux variables are just declared but never used in the execution chain.

public Mono<Post> findPostByIdShowComments(String id) {
    return postRepo
            .findById(id)
            .switchIfEmpty(postNotFoundException())
            .flatMap(postFound -> commentService
                    .findCommentsByPostId(postFound.getId())
                    .collectList()
                    // it might be also a good option (and a cleaner operator here) instead of .flatMap(...)
                    //.doOnSuccess(comments -> postFound.setListComments(comments))
                    //.thenReturn(postFound)
                    .flatMap(comments -> {
                        postFound.setListComments(comments);
                        return Mono.just(postFound);
                    })
            );
}

Now we injected comments flow and their injection into found post as a part of the main flow.