title: Is there an operator that works as concatMap but with more than one inner observables
body: <p>I'm using an observable to query my DB.
  This observable will return an array with all the matching objects found.
  My problem is that I want to map the observable with more details that I will retrieve from another API. </p>
  
  <p>I tried concatMap but it only let me nest 1 observable inside the initial observable</p>
  
  <pre class="lang-js prettyprint-override"><code>const usersAPI$: Observable&lt;Users[]&gt; = //  something
  const assistsAPI$: Observable&lt;Assists[]&gt; = //  something
  
  const modifiedAssists$ = assistsAPI$.find().pipe(
    concatMap(assists =&gt;
      assists.map(assist =&gt; usersAPI$.get(assist.userID)
         .pipe(
            map(user =&gt; 
              assist = {...assist}, ...{userName: user.userName}
            )
          )
      )
    )
  );
  </code></pre>
  
  <p>You can see a similar working example here, on stackblitz,
  <a href="https://stackblitz.com/edit/rxjs-concatmap-issue-stackoverflow" rel="noreferrer">https://stackblitz.com/edit/rxjs-concatmap-issue-stackoverflow</a>
  where the "result" is the correct way of using concatMap and the
  "result2" is the non working way that I expected to work</p>