title: Firebase - Firestore - get key with collection.add()
body: <p>I am facing a problem with the new Firestore from Firebase.</p>
  
  <p><strong>Situation:</strong> I have a <code>collection('room')</code></p>
  
  <p>I create room with <code>collection('room').add(room)</code></p>
  
  <hr>
  
  <p><strong>What I'm trying to do:</strong> I need to update a room.</p>
  
  <p>For this, I use: <code>collection('room').doc(ROOM_ID).update(update)</code></p>
  
  <p>So I need to add ROOM_ID in the document in my collection:</p>
  
  <pre><code>|room
      ROOM_ID
          id:ROOM_ID,
          someContent: ForTheQuery
  </code></pre>
  
  <hr>
  
  <p>Is there a possible way to achieve that?</p>
  
  <p>An alternative is to create myself a generated ID with:</p>
  
  <pre><code>collection('room')
  .doc(someId)
  .set({
      id: someId,
      someContent: ForTheQuery
  });
  </code></pre>
  
  <p>but i want to avoid it.</p>