title: Pass value from Mono/Flux to methods
body: <p>I'm new to reactive programming concept. I'm following the "Learning Spring Boot 2.0" and the described simple concepts/examples are understandable. But I don't get how to use Mono/Flux in more complex use cases. 
  Some example with  with spring boot, mongo and project reactor</p>
  
  <p>My model</p>
  
  <pre><code>User
  @Id
  private String id;
  private String login;
  
  Comment
  @Id
  private ObjectId _id;
  private String content;
  private String ownerLogin;
  </code></pre>
  
  <p>so this simple example to get comments by owner works fine</p>
  
  <pre><code>@GetMapping(params = "login")
  @ResponseBody
  public Flux&lt;Comment&gt; getAllCommentsByLogin(@RequestParam("login") String login) {
      return commentRepository.findByOwnerLogin(login);
  };
  </code></pre>
  
  <p>But if I would slightly change the model to store owner by entity id, it would be not so ease to retrieve the comments by owner</p>
  
  <pre><code>Comment
  @Id
  private ObjectId _id;
  private String content;
  private String ownerId;
  </code></pre>
  
  <p>my intention is to make rest controller easy to use by end user and first find the user entity by login and if exists all user comments</p>
  
  <pre><code>@GetMapping(params = "login")
  @ResponseBody
  public Flux&lt;Comment&gt; getAllCommentsByLogin(@RequestParam("login") String login) {
      return commentRepository.findByOwnerId(userRepository.findByLogin(login).map(user2 -&gt; user2.getId())
  };
  </code></pre>
  
  <p>this soulution is obviously wrong, but I don't know if the whole approach is wrong or only this method.</p>