Structure Realtime Database data for querying nested fkeys. See https://groups.google.com/d/msgid/firebase-talk/8c3ff322-c106-490e-8de1-d121857c5506%40googlegroups.com.

data_structure.json

{
  "albums": {
     "key1": {
         "owner": "uid1",
         "subscribers": {
             "uid1": true,
             "uid2": true
         }
     }
  }
}

query.js

// Since we've used the unique identifier instead of a push id, we can query as follows

const query = firebase.database().ref('albums').orderByChild('subscribers/uid1').equalTo(true);