RxJs: How to conditionally chain observable of BehaviorSubject? - conditional-statements

Question

I've got an observable data service (UserService) that returns the currently logged in user. I followed this tutorial - https://coryrylan.com/blog/angular-observable-data-services, which describes using a BehaviorSubject to return a default currentUser immediately, then emit the real currentUser once it's loaded or altered. The service is basically like this...
private _currentUser: BehaviorSubject<User> = new BehaviorSubject(new User());
public currentUser: Observable<User> = this._currentUser.asObservable();
constructor(private http: Http) {}
loadUser() { // app.component onInit and login component call this
return this.http.get('someapi.com/getcurrentuser')
.map(response => <User>this.extractData(response))
.do(
(user) => {
this.dataStore.currentUser = user;
this._currentUser.next(Object.assign(new User(), this.dataStore).currentUser);
},
(error) => this.handleError(error)
)
.catch(error -> this.handleError(error));
}
I'm having problems whenever a user hits F5 to reload the entire spa. When a consuming component subscribes to the currentUser on the UserService, it immediately receives a default user while the UserService waits for an api call to receive the actual user. The moment that api call finishes, the real user is emitted by UserService and all the subscribers get the real user. The first value emitted by the BehaviorSubject, however, is the default value and it always has an id of "undefined", so we can't make our next api call yet. In fact, when the real user comes through and I CAN make a valid call using the user.id, the chained subscription never happens and I don't get the values out of the response.
I know I'm doing something stupid, but I haven't figured out exactly what yet. I just stumbled across concatMap, but I'm not sure how to use it. While I pursue that, I'd like to know why the below code doesn't work. I particularly want to know why the subscribe never fires, even after the real user comes in, just to help my newbie understanding of Observables.
this.userService.currentUser
.flatMap((user) => {
this.user = user;
// Need to NOT call this if the user does not have an id!!!
this.someOtherService.getSomethingElse(user.id); // user.id is always undefined the first time
})
.subscribe((somethingElse) => {
// This never gets called, even after the real user is emitted by the UserService
// and I see the getSomethingElse() call above get executed with a valid user.id
this.somethingElse = somethingElse;
});

Answer 1

If you want to ignore user instances that do not have an id, use the filter operator:
import 'rxjs/add/operator/filter';
this.userService.currentUser
.filter((user) => Boolean(user.id))
.flatMap((user) => {
this.user = user;
this.someOtherService.getSomethingElse(user.id);
})
.subscribe((somethingElse) => {
this.somethingElse = somethingElse;
});
Regarding "why the subscribe never fires", it's likely due to an error arising from the undefined id. You only pass a next function to subscribe, so any errors will be unhandled. And if an error occurs, the observable will terminate and will unsubscribe any subscribers - as that is how observables behave - so any subsequent users with defined id properties will not be received.