title: Firestore collection group query on document id
body: <p>I am trying to run the following query:</p>
  <pre><code>this.db.firestore
          .collectionGroup('members')
          .orderBy('dateModified', 'desc')
          .where(firebase.firestore.FieldPath.documentId(), '==', uid)
  </code></pre>
  <p>But I get the error:</p>
  <blockquote>
  <p>Invalid query. When querying a collection group by
  FieldPath.documentId(), the value provided must result in a valid
  document path, but 'X6yL5Ko55jSMWhhoQUO91xVYdEH3' is not because it
  has an odd number of segments (1).</p>
  </blockquote>
  <p>Is the only way around this to add the document id to the document?
  This is not ideal as I have a lot of existing data...
  The document id is the uid of the firebase user.</p>