title: Android Firestore search collection where document is Unique id
body: <p>Is it possible with Firestore to retrieve the data within a document where the name of the document is unknown or a unique id. Like in the linked image. The ID is unknown whereas the name is known, is it possible to get the ID in this instance?
  <img src="https://i.stack.imgur.com/yB2iC.jpg" alt="Database structure example"></p>
  
  <pre><code>db.collection("users").document(??).collection("John Doe");
  </code></pre>