title: How to get key from a Firebase list through Angularfire 5.0
body: <p>I was trying to upgrade my Angularfire 4 code to its latest version. There are some breaking changes as it no longer emits $key. The official documentation states to manage it by nesting map operators that I am struggling hard to understand -</p>
  
  <pre><code>constructor(afDb: AngularFireDatabase) {
    afDb.list('items').snapshotChanges().map(actions =&gt; {
      return actions.map(action =&gt; ({ key: action.key, ...action.payload.val() }));
    }).subscribe(items =&gt; {
      return items.map(item =&gt; item.key);
    });
  }
  </code></pre>
  
  <p>How is this nested map working? Why can't I perform the same operation in single map operator?</p>