I’ve this Angular service that I’m trying to mock:

@Injectable({
  providedIn: 'root',
})
export class NotificationsService {
  private _notifications = new Subject<Notification>();
  public notifications: Observable<Notification> = this._notifications.asObservable();
  public crudNotifications: Observable<CrudNotification> = this._notifications.asObservable().pipe(
    filter(notification => notification.notificationType === 'crud-notification'),
    map(notification => notification as CrudNotification)
  );
//....
}

I just migrated this to Jest and I’m trying to figure out how to mock the return a custom observable when my tested service tries to access to NotificationsService.crudNotifications

Currently, I’ve this:

describe('OrganizationUsersService', () => {
  let organizationUsersService: OrganizationUsersService;
  let crudNotifications!: Subject<CrudNotification>;
  let notificationServiceMock!: Spy<NotificationsService>
  let storeMock!: Spy<Store>;
  let httpTestingController! :HttpTestingController;
  
  beforeEach(()=>{
    

    TestBed.configureTestingModule({
      providers:[
      {provide: NotificationsService, useValue: createSpyFromClass(NotificationsService)},
      {provide: Store, useValue:     createSpyFromClass(Store)  },
      {provide: Router, useValue:     createSpyFromClass(Router)  },
      provideHttpClient(),
      provideHttpClientTesting(),
      ]
    })
    httpTestingController = TestBed.inject(HttpTestingController);
    notificationServiceMock = TestBed.inject<any>(NotificationsService);
    storeMock = TestBed.inject<any>(Store);

    //Common mocked stuff behavior for every methods here    
    crudNotifications = new Subject<CrudNotification>();
    notificationServiceMock.crudNotifications = crudNotifications.asObservable();

    storeMock.dispatch.mockReturnValue(of());
    storeMock.select.mockReturnValue(of({ id: 'some-id' }));

    //Finally create the service to test:    
    organizationUsersService = TestBed.inject(OrganizationUsersService);
  })

    fit('should refresh the store when the backend inform a new role attribution has been added for the current organization id', fakeAsync(() => {
    //Arrange
    organizationUsersService.initialize(); //Required to register

    //Act
    crudNotifications.next({
      dataType: 'organization.role-attribution',
      operationType: 'add',
      id: 'some-id',
      notificationType: 'crud-notification',
    });
    tick();

    //Assert
    httpTestingController.expectOne(environment.backendUrl + 'some/stuff').flush({});
    httpTestingController.expectOne(environment.backendUrl + 'some/other/stuff').flush({});

    expect(storeMock.dispatch).toHaveBeenCalledWith(expect.any(SetLoadingAction));
    organizationUsersService.dispose();
  }));
}));

This doesn’t work because the observable I’m trying to set doesn’t have the Spy methods on.
I tried this:

notificationServiceMock.crudNotifications.mockReturnValue(crudNotifications);

but the mockReturnValue method doesn’t exists on public fields.

Is there a way to have mocked value on public fields?