title: Firestore document/subcollection is not existing
body: <p>I have this db structure:</p>
  
  <pre><code>Firestore-root
  |
  --- sites (collection)
  |     |
  |     --- common (document)
  |            |
  |            --- //templates (collection)
  |            |
  |            --- //menus (collection)
  |            |
  |            --- //articles (collection) &lt;----
  |     --- other (document)
  |            |
  |            --- //articles (collection)
  </code></pre>
  
  <p>When I try to add articles to the db (shown by the arrow) the "common" and the "other" - document is in italic and therefore doesn't exist.</p>
  
  <p>My code when i try to add: priority is common, type is articles.</p>
  
  <pre><code>def documentReference = firestoreClient.databaseReference.collection(siteName).document(priority).collection(TYPE).document(key)
  documentReference.set(article)
  </code></pre>
  
  <p>this is the console:
  <img src="https://i.stack.imgur.com/UkUR4.png" alt="firestore console image"></p>
  
  <p>Is this a bad way to structure my db or is there a quick fix?</p>
  
  <p>I have tried to create the sub collections first but without any luck!</p>