title: Append element to Firebase Array
body: <p>How I could append an element to an array like that:
  <a href="https://i.stack.imgur.com/12yM9.png" rel="noreferrer"><img src="https://i.stack.imgur.com/12yM9.png" alt="enter image description here"></a></p>
  
  <p>Using this code I'm overriding the old data:</p>
  
  <pre><code>let toUpdate = [book.id]
  self.refUsers.child(localUser.key!).child("booksPurchased").setValue(toUpdate, withCompletionBlock: { (error, _) in
  </code></pre>