How To Do Recursive Observable Call in RxJava?

I am quite new to RxJava (and Reactive paradigm in general), so please bear with me.

Suppose I have this News and this nested Comment data structure:

public class News {
  public int id;
  public int[] commentIds; //only top level comments
  public News(int id, int[] commentIds) {
    this.id = id;
    this.commentIds = commentIds;
  }
}

public class Comment {
  public int id;
  public int parentId; //ID of parent News or parent comment
  public int[] childIds;
  public Comment(int id, int parentId, int[] childIds) {
    this.id = id;
    this.parentId = parentId;
    this.childIds = childIds;
  }
}

and suppose I have this API endpoint:

getComments(int commentId) //return Observable<Comment> for Comment with ID commentId

Now, let's assume:

getComments(1); //will return Comment(1, 99, [3,4])
getComments(2); //will return Comment(2, 99, [5,6])
getComments(3); //will return Comment(3, 1, [])
getComments(4); //will return Comment(4, 1, [])
getComments(5); //will return Comment(5, 2, [])
getComments(6); //will return Comment(6, 2, [])

Now, if I have `News n = News(99, [1,2])`, how do I get all of its children comment recursively? i.e. to get comments with ID [1,2,3,4,5,6]?

I have searched and stumbled upon this: https://jkschneider.github.io/blog/2014/recursive-observables-with-rxjava.html

This is the recursion function:

public class FileRecursion {
    static Observable<File> listFiles(File f) {
        if(f.isDirectory())
            return Observable.from(f.listFiles()).flatMap(FileRecursion::listFiles);
        return Observable.just(f);
    }

    public static void main(String[] args) {
          Observable.just(new File("/Users/joschneider/Desktop"))
                  .flatMap(FileRecursion::listFiles)
                  .subscribe(f -> System.out.println(f.getAbsolutePath()));
    }
}

It shows an example on how to do recursive observable calls, but the inner function (f.listFiles()) is a blocking operation (doesn't return another Observable). In my case, the inner function (getComments) is a non-blocking function that returns another Observables. How do I do that?

Any help will be much appreciated.

This does practically the same thing described in the article:

Observable<Comment> getInnerComments(Comment comment) {
    if (comment.childIds.length > 0)
        return Observable.merge(
                Observable.just(comment),
                Observable.from(comment.childIds)
                        .flatMap(id -> getComments(id))
                        .flatMap(this::getInnerComments));
    return Observable.just(comment);
}

public static void main(String[] args) {
    getComments(1)
          .flatMap(this::getInnerComments)
          .subscribe(c -> System.out.println(comment.toString()));
}

I start with the comment with id = 1, then I pass it to getInnerComments(). The getInnerComments() checks if the comment has children. If it does, it iterates over every child id (Observable#from) and loads every child with your getComments(int) API. Then every child is passed to the getInnerComments() to do the same procedure. If a comment doesn't have children, it is immediately returned using Observable#just.

This is pseudo-code and it wasn't tested, but you should get the idea.

Below is an example of how to get all comments and then aggregate them to one List<Comment>.

getNews(99)
        .flatMap(news -> Observable.from(news.commentIds))
        .flatMap(commentId -> getComments(commentId))
        .flatMap(comment -> getInnerComments(comment))
        .toList()
        .subscribe(commentList -> { });