title: RxJS: SwitchMap for Array of Strings
body: <p>Following use case: A user can join 0...* groups. Each group has an ID and contains 0...* posts.</p>
  
  <p>I subscribe to an Observable (to get the groups of the user he joined) and this returns an array of strings (the group IDs).</p>
  
  <pre><code>const groupIds$ = of(['a', 'b', 'c']);
  </code></pre>
  
  <p>If I only had one I would now use <code>switchMap</code> and return this new observable and subscribe to it to get the posts from the group.</p>
  
  <p>But so I have an array and so this isn't working.  Does anyone has an idea which RxJS operator(s) can achieve this to get the posts from all groups?</p>
  
  <p>Or does no operator for such use case exist and I have to do it separately at subscribe?</p>