title: Combine RXJS observable results into one
body: <p>I have two nodes in my database: </p>
  
  <pre><code>users: {user1: {uid: 'user1', name: "John"}, user2: {uid: 'user2', name: "Mario"}}
  homework: {user1: {homeworkAnswer: "Sample answer"}}
  </code></pre>
  
  <p>Some users might or might not have a homework. </p>
  
  <p>I want to obtain a list of all users with each user's homework data with a single call and subscribe. What would be the best way to achieve this?</p>
  
  <p>Here is how the list obtained should look like for the example above:</p>
  
  <pre><code>[{uid: 'user1', name: "John", homework: {homeworkAnswer: "Sample answer"}}, {uid: 'user2', name: "Mario"}]
  </code></pre>
  
  <p>These are my two observables for <code>users</code> and <code>homework</code>:</p>
  
  <pre><code>let usersObservable = this.af.getObservable(`users/`);
  let hwObservable = this.af.getObservable(`homework/`);
  </code></pre>