getComments(mealId: number): Observable<Comment[]> {
    return this.http.get<Comment[]>(`${this.baseUrl}${this.mealUrl}/${mealId}/${this.commentUrl}`).pipe(
      map(results => results['_embedded'].comments),
      mergeMap(comments => comments.length
        ? forkJoin(
          comments.map(
            comment => forkJoin(
              of(comment),
              this.http.get<User>(`${comment._links.sender.href}`)
            ).pipe(
              map(data => { return { ...data[0], author: data[1] } })
            )
          )
        )
        : of(null)
      )
    );
  }