title: Firestore query where map contains string
body: <p>Data structure:</p>
  
  <pre><code>houses (collection)
    name (string)
    users (map)
      90c234jc23 (map)
        percentage: 100% (string/number)
  </code></pre>
  
  <p>Rules:</p>
  
  <pre><code>allow read: request.auth.uid in resource.data.users;
  </code></pre>
  
  <p>The problem is when I try to query houses which user owns:</p>
  
  <pre><code>FirebaseFirestore.getInstance().collection(House.COLLECTION)
  //                .whereArrayContains(House.USERS_FIELD, currentUser.getUid()) // does not work
                  .whereEqualTo("users." + currentUser.getUid(), currentUser.getUid()) // does not work either
                  .get()
  </code></pre>
  
  <p>No result are returned.</p>