我正在构建一个简单的市场网络应用程序，用户在那里发布他们的产品。我想要获取每个产品的用户所有者。请告诉我如何使用可观察对象执行Firebase内部联接！

productsRef: AngularFireList<any>;

  ///
 constructor(
    private db: AngularFireDatabase,
    private userService: UserService

  ) {
    this.productsRef = db.list('/products');
  }

getProductsList() {
    const product$ = this.productsRef.valueChanges();
    // 1st observable to get the products
    const productsResults$ = product$.pipe(
      switchMap(products => products)
      )
    );

    productsResults$.subscribe(
      console.log,
      console.error,
      () => console.log('completed httpResult$')
    );

   // 2nd observable  to get the user 
    const user$ = this.userService.getUserById(Id);

    productsResults$.subscribe(
      console.log,
      console.error,
      () => console.log('completed httpResult$')
    );

}

我得到了以下2个结果。
产品

[
  {
    "key": "-TM2Y6vBk70rgKZ3zTUAw",
    "name": "T-shirt",
    "size": "M",
    "user": "-NDNPe47CDTbjmwGgW_3",
  }, 
]

用户

{
    "key": "-NDNPe47CDTbjmwGgW_3",
    "username": "Alex199",
  },

我想要的最终结果

{
    "key": "-TM2Y6vBk70rgKZ3zTUAw",
    "name": "T-shirt",
    "size": "M",
    "user": {
       "key": "-NDNPe47CDTbjmwGgW_3",
        "username": "Alex199",
     }, 
},

我如何使用可观察对象来实现这一点？