What is the best way to merge queries by condition?

Question

Angular

Good afternoon.
The essence of the question is this:
1. A request is made to the server post = getPostByName(name)
2. The post id is searched for tags post.tags = getTagsByPostId(post.id)
3. If the post has comments, the post.comments comments are searched = post.hasComments ? getCommentsByPostId(post.id) : null
If the first two points can be done via mergeMap , then the third point is unclear how to solve.

this.postService.getPostByName(name)
.pipe(
  mergeMap( 
    post => this.postService.getTagsByPostId(post.id),
      (post, tags) => {
        post.tags = tags;
        return post;
    }
),
.subscribe( post => {
  this.posts.push(new Post(post));
});

What is the best way to add another conditional query and wait for it to complete? The problem is that all the data must come to the constructor already formed, i.e. must wait for two (or three) queries to complete before passing to the new Post(post) constructor

Answer 1

well something like this

this.postService.getPostByName(name).pipe(
  mergeMap(post => forkJoin([
    of(post),
    this.postService.getTagsByPostId(post.id),
    iif(() => post.hasComments(), this.getCommentsByPostId(post.id), of(null)),
  ]))
)
  .subscribe(([post, tags, comments]) => {...})

It is better to put the components into custom statements so that the code can be read

Answer 2

this.postService.getPostByName(name)
  .pipe(
    switchMap(post => this.postService.getTagsByPostId(post.id)
      .pipe(
        tap(tags => post.tags = tags),
        mapTo(post),
        filter(() => post.hasComments),
        switchMap(() => this.postService.getCommentsByPostId(post.id)),
        tap(comments => post.comments = comments),
        mapTo(post),
      )
    ),
  )