How to return observable from subscribe

Question

I'm trying to return an observable when I get a certain value in a subscriber, but I fail miserably.

This is the code:

canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot):Observable<boolean> {
    // get route to be activated
    this.routeToActivate = route.routeConfig.path;

    // get user access levels        
    return this._firebase.isUserAdmin          <-- returns Subscription, not Observable
        .map(user => user.access_level)
        .subscribe( access => {
           // I need to return an observable here
        });
}

There are not many resources on observables in angular 2, so I don't know where to start. Can anyone help with this please?

UPDATE -> Working Version

canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot):Observable<boolean> {
            // get route to be activated
            this.routeToActivate = route.routeConfig.path;

            // get user access levels        
            return this._firebase.isUserAdmin
                .map(user => {
                    let accessLevel = user.access_level;

                    if (accessLevel === 'admin' ) {
                        return true;
                    }

                }).first();
        }

Answer 1

You can't return an observable from subscribe but if you use map instead of subscribe then an Observable is returned.

canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot):Observable<boolean> {
    // get route to be activated
    this.routeToActivate = route.routeConfig.path;

    // get user access levels        
    return this._firebase.isUserAdminObservable
        .map(user => {
           // do something here
           // user.access_level;
           return true;
         })
        .first(); // for the observable to complete on the first event (usually required for `canActivate`)
        // first needs to be imported like `map`, ...
}

canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot):Observable<boolean> {
    // get route to be activated
    this.routeToActivate = route.routeConfig.path;

    let subject = new Subject();
    // get user access levels        
    this._firebase.isUserAdminObservable
        .map(user => {
          let accessLevel = user.access_level; 
          if (accessLevel === 'admin' ) { 
            subject.emit(true); 
            subject.complete();
          } 
          return user;
        });
     return subject;
}

Answer 2

Couldn't we just use pipe with map from import { map } from 'rxjs/operators';?

import { map } from 'rxjs/operators';

@Injectable({
  providedIn: 'root'
})
export class SearchHostService {
  constructor(private readonly http: HttpClient) {}

  searchForHosts(searchString: string): Observable<Employee[]> {
    return this.http
      .get<Employee[]>('./assets/host-users.json')
      .pipe(
        map(employees =>
          employees.filter(({ displayName }) =>
            displayName.toLowerCase().startsWith(searchString),
          ),
        ),
      );
  }
}