{
  "data": {
    "index_1": {
      "xxx": {
        "value_1": "Some data",
        "value_2": true
      },
    "index_2": {
      "xxx": {
        "value_1": "Some data",
        "value_2": true
      },
    "index_3": {
      "xxx": {
        "value_1": "Some data",
        "value_2": true
      },
   },
   "keys": {
     "user1": {
       "index_1": true,
       "index_2": true
     },
     "user2": {
       "index_1": true,
       "index_3": true
     }

   }
}

public class Data {
    private String value_1;
    private Boolean value_2;
    ...
    getters/setters
    ...
}

"data": {
  "index_1": {
    "value_1": "Some data",
    "value_2": true
  }
}

keyQuery = FirebaseDatabase.getInstance()
                           .getReference("keys")
                           .orderByKey();
dataRef = FirebaseDatabase.getInstance().getReference("data");
FirebaseRecyclerOption<Data> options = new FirebaseRecyclerOptions.Builder<Data>().
        .setIndexedQuery(keyQuery, dataRef, Data.class)
        .build();