title: Firestore collection group query on documentId
body: <p>In my scenario a user can "like" the profile of another user. As a result I have a subcollection called "likedBy" for each user, where I create a document for a specific user, e.g.: </p>
  
  <blockquote>
    <p>users(col) -> User A(doc) -> likedBy(col) -> User B (doc), User C (doc)</p>
  </blockquote>
  
  <p>So in the rare scenario of a user being deleted, I want to delete all the likes issues by that user.</p>
  
  <p>I am just not sure if this is even possible (a workaround could be to just save the userId again in said document and query for that).</p>
  
  <p>What I am basically looking for is something like this:</p>
  
  <pre><code>db.collectionGroup('likedBy').where(firebase.firestore.FieldPath.documentId(), '==', "User A").get();
  </code></pre>
  
  <p>The problem is, that I can not create an index for the documentId in the Firestore console.</p>