title: Firebase Firestore: Append/Remove items from document array
body: <p>I am trying to append/remove items from an array inside of a Firestore Document but every time the entire array is replaced instead of the new value being appended. I have tried both of the following:</p>
  
  <pre><code>batch.setData(["favorites": [user.uid]], forDocument: bookRef, options: SetOptions.merge())
  
  batch.updateData(["favorites": [user.uid]], forDocument: bookRef)
  </code></pre>
  
  <p>I know that instead of an array I can use an object/dictionary but that would mean storing additional data that is irrelevant (such as the key), all I need is the ID's stored inside the array. Is this something that is currently possible in Firestore?</p>