AngularFire2 Auth service that merges custom data from the database

auth.service.ts

import { Injectable } from '@angular/core';
import { AngularFireAuth } from "angularfire2/auth";
import { AngularFireDatabase } from "angularfire2/database";
import * as firebase from 'firebase';

import { BehaviorSubject } from 'rxjs/BehaviorSubject';

export class User {
  uid: string;
  username:string;
  email:string;

  constructor(uid, username, email) {
    this.uid = uid
    this.username = username || ""
    this.email = email || ""
  }
}



@Injectable()
export class AuthService {

  _userSource = new BehaviorSubject(null);
  currentUser = this._userSource.asObservable();


  constructor(public afAuth: AngularFire,
              private db: AngularFireDatabase,
              private router:Router) {

            afAuth.authState.subscribe( (auth) => {
              this.buildUser(auth)
            });

          }

  private buildUser(auth) {
    // retrieves custom user data from the realtime database
    if (auth) {
      this.db.object(`users/${auth.uid}`).subscribe((data) => {
          let user = new User(data.$key, data.username, data.email)
          this._userSource.next(user)
        })
    }
  }