title: How to get DocumentSnapshot id as String?
body: <p>How can i get the id of a document in Firestore?</p>
  
  <pre><code>final String PostKey = db.collection("Anuncio").document().getId();
  </code></pre>
  
  <p>I'm trying on this way, but it return a new id. How can get de id of the a document that already exit?</p>