AngularFire2 Firebase Observable never ends (list is empty) - firebase

Question

I'm trying to query an empty firebase list. The problem is that the observable method subscribe never finish and I can't show to user that ddbb list is empty.
The function getUserAppointmentsByDate(...) is calling getUserAppointments(...), where this.database.list('/appointment/users/' + user_uid) is an empty firebase list for the input user (user_uid).
how should I manage an empty query to firebase?
thanks in advance!
getUserAppointmentsByDate(user_uid: string, start: string, end: string) {
if (typeof (user_uid) == "undefined" || typeof (start) == "undefined" || typeof (end) == "undefined") {
console.error("invalid argument for getPatientReport");
return;
}
return this.getUserAppointments(user_uid)
.map(
(appointment) => {
return appointment
.filter((appointment) => {
var appointmentStart = new Date(appointment.start);
var startFilter = new Date(start);
var endFilter = new Date(end);
//Filter old, not cancelled and not deleted
return (appointmentStart.getTime() < endFilter.getTime())
&& (appointmentStart.getTime() > startFilter.getTime())
&& (appointment.status != AppointmentStatus.CANCELLED);
});
})
}
getUserAppointments(user_uid: string): any {
return this.database.list('/appointment/users/' + user_uid) //*THIS IS AN EMPTY LIST
.mergeMap((appointments) => {
return Observable.forkJoin(appointments.map(
(appointment) => this.database.object('/appointment/list/' + appointment.$key)
.take(1)))
})
}

Answer 1

As the this.database.list('/appointment/users/' + user_uid) return a empty array. Observable.forkJoin(appointments.map( complete without emit any value (that is the expected way of forkJoin works). In this case, you have two options, handling in the complete function.
.subscribe(
res => console.log('I got values'),
err => console.log('I got errors'),
// do it whatever you want here
() => console.log('I complete with any values')
)
or handle in an if statement:
import { of } from 'rxjs/observable/of';
...
return this.database.list('/appointment/users/' + user_uid)
.mergeMap((appointments) => {
if (appointments.length === 0) return of([]);
return Observable.forkJoin(appointments.map(
(appointment) => this.database.object('/appointment/list/' + appointment.$key)
.take(1)))
})