title: RxJava - fetch every item on the list
body: <p>I have a method that returns an <code>Observable&lt;ArrayList&lt;Long&gt;&gt;</code>, which are ids of some Items. I'd like to go through this list and download every Item using another method that returns <code>Observable&lt;Item&gt;</code>.</p>
  
  <p>How would I do this using RxJava operators?</p>