Performing two firebase queries to inner join 2 lists - angularjs

Question

I'm building a simple marketplace web app where the user posts their products. I want to get the user owner of each product. Please tell me how do I perform Firebase inner joins using observables!
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$.subscribe(
console.log,
console.error,
() => console.log('completed httpResult$')
);
}
i get the 2 following results.
Products
[
{
"key": "-TM2Y6vBk70rgKZ3zTUAw",
"name": "T-shirt",
"size": "M",
"user": "-NDNPe47CDTbjmwGgW_3",
},
]
User
{
"key": "-NDNPe47CDTbjmwGgW_3",
"username": "Alex199",
},
the end result i want
{
"key": "-TM2Y6vBk70rgKZ3zTUAw",
"name": "T-shirt",
"size": "M",
"user": {
"key": "-NDNPe47CDTbjmwGgW_3",
"username": "Alex199",
},
},
How can I achieve this using observable?

Answer 1

There is no Inner Join clause in NoSQL databases. So if you want to read some data at a location based on what you read at another location, then you have to perform two separate requests. There is no way you can get that data in a single go.
Alternatively, if you don't want to perform two separate queries, then you can save user data right inside the product node, or vice versa. This practice is called denormalization and is a common practice when it comes to NoSQL databases like Realtime Database. For a better understanding, I recommend you see this video.
If you consider at some point in time try using Cloud Firestore, then I recommend you read my answer from the following post:
What is denormalization in Firebase Cloud Firestore?