title: Firestore - Possible to query by array-not-contains?
body: <p>I am aware it would be very difficult to query by a value that does not exist in an array but is there a way to do this without doing exactly that?</p>
  
  <p>Here is my scenario - I have a subscription based service where people can opt in and "follow" a specific artist. In my backend, this creates a subscription doc with their followerId, the id of the artist they want to follow (called artistId), and an array called pushed. The artist can add new releases, and then send each follower a notification of a specific song in the future. I would like to keep track of which follower has been pushed which release, and this done in the aforementioned pushed array. I need a way to find which followers have already been pushed a specific release and so...</p>
  
  <p>I was thinking of combining two queries but I am not sure if it is possible. Something like:</p>
  
  <ol>
  <li><code>db.collection('subscriptions').where('artistId', '==', artistId)</code></li>
  <li><code>db.collection('subscriptions').where('artistId', '==', artistId).where('pushed', 'array-contains', releaseId)</code></li>
  </ol>
  
  <p>And then take the intersection of both query results and subtract from the 1st query to get the followers that have not been pushed a specific release.</p>
  
  <p>Is this possible? Or is there a better way?</p>