isCodeExist(code: string): Observable<Job[]> {
  return <Observable<Job[]>>this.afs
    .collection(this.jobsCollection, ref => ref
      .where('code', '==', code)
      .limit(1)
    )
    .valueChanges()
    .pipe(
      flatMap(jobs => jobs),
      catchError(err => throwError(err)
      )
    );
}