Angular Fire - Firebase Realtime DB : Joining two collections

Question

I am having a simple data structure

Dashboard:

User Collection:

I am reading the dashboard collection and for each UID , I want to query the user table and get the user details.

I tried several option , none worked. But the follow method gave the nearest Results. however the suer information is not coming.

 public getDashboard(practiceID)
  {
    const path = 'dashboard/'+practiceID;
    return this.db.list(path).snapshotChanges().pipe(map(rag=>{
        return rag.map(a=>{
            
            const payload:IRAG = a.payload.val();
            const uid = a.key;
            const upath = 'users/'+uid;
            payload.user = this.db.list(upath).valueChanges()
                                              .pipe(map(u=> {  return u}));
            return payload;
        })}))
     
  }

Any help would be deeply appreciated