title: RxJava combine/merge object with list
body: <p>I have 2 observables, one returns 1 element and the other one returns multiple elements.
  My goal is to merge them together, <strong>without blocking</strong>, to build an object like this :</p>
  
  <pre><code>class Combined {
      Server server;
      List&lt;Client&gt; client;
  }
  
  Observable&lt;Server&gt; server = getServerById("1"); //returns an observable of 1 server
  Observable&lt;Client&gt; clients = getClientsByServerId("1"); //returns an observable of 0 to n clients
  
  Observable&lt;Combined&gt; combined = //...
  </code></pre>
  
  <p>I tried zip, merge and combine, none of them seems to be the solution.</p>