Check for a value in many child nodes in firebase real-time database

Question

I want to retrieve the text 'Private Group' from the child 'group' IF the child 'contacts' has the value '9aIMkiMa0bSuMLjUk3R5bLpnoQS2'. I also want to check for the same value from the other child nodes of the parent child 'questions posts'. I have struggled with it but in vain. This is the code that I have been using, but It has only produced errors.

final DatabaseReference groupsRef = FirebaseDatabase.getInstance().getReference().child("questions posts");
    Query query = groupsRef.orderByChild("group").orderByChild("contacts");
    query.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot snapshot) {
            arrayList.clear();
          if (snapshot.exists() && snapshot.hasChild(onlineUserId)){
              arrayList.add(groupsRef.getKey().toString());
              arrayAdapter.notifyDataSetChanged();
          }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError error) {

        }
    });

This is the image of the database structure

No, in the other nodes, it can be different. It is a value typed by a user. — willy, Sep 28 '20 at 08:58

score 1 · Accepted Answer · answered Sep 28 '20 at 15:14

A Firebase query operates on a flat list of nodes, and can only contain a single unknown key.

From what I can tell you have at least two levels of unknown keys (-MH...Ez12, and 9alMk....QS2) and possibly three (Private Group). There is no way to query this structure with a database query, and you will need to come up with a different data structure to allow your use-case.

For example, if you want to allow finding the posts for a given contact ID, consider storing exactly that relationship:

"contactsToPosts": {
  "$contactId": {
    "$postId": true
  }
}

For more on this sort data modeling trade-off, see:

Check for a value in many child nodes in firebase real-time database

1 Answers1