get data from 2 sources - asynchronous

Question

I'm learning programming and async using promise or observable and it's a bit hard to understand
I have 2 data sources, posts (with user id who posted it), and users, what I want to achieve is simply to display post with some users info like image, name, etc, how to I proceed?
This is how I get my sources:
allUsers() {
return new Promise((resolve, reject) => {
this.rootRef.child('users').once('value', snapshot => {
const data = snapshot.val()
const users = Object.keys(data).map(id => new User(id, data[id]))
resolve(users)
})
})
}
for posts
allPosts(): Subject<Post[]> {
const postsObservable = new BehaviorSubject<Post[]>(null)
this.rootRef.child('posts').on('value', snapshot => {
const data = snapshot.val()
const posts = Object.keys(data)
.map(id => new Post(id, data[id]))
.sort((lhs, rhs) => rhs.date.getTime() - lhs.date.getTime())
postsObservable.next(posts)
})
return postsObservable
}
Do I need to add user data like image and name when a user posts something, or is it possible to do it this way.

Answer 1

I would call your getUsers method when posts are received. This way you would have both data. You would call the next method after having set the user hints into posts:
allPosts(): Subject<Post[]> {
const postsObservable = new BehaviorSubject<Post[]>(null)
this.rootRef.child('posts').on('value', snapshot => {
const data = snapshot.val()
const posts = Object.keys(data)
.map(id => new Post(id, data[id]))
.sort((lhs, rhs) => rhs.date.getTime() - lhs.date.getTime())
this.getUsers().then(users) => {
posts.forEach((post) => {
// Set user hints into posts
});
postsObservable.next(posts);
});
});
return postsObservable;
}