Angular testing - ngBootstraps typeahead

I am currently using a autocomplete mechanism (typeahead) of ngBootstrap. Now I want to unit test if a method is called on every sequence of an input event. The error on my test case is currently: Cannot read property 'pipe' of undefined

Html:

<input id="locationEdit" type="text" class="form-control"
         [(ngModel)]="user.location" name="location [ngbTypeahead]="search"/>

Component:

public ngOnInit() {
    this.search = (text$: Observable<string>) =>
      text$.pipe(
        tap(() => {
          this.isSearching = true;
          this.searchFailed = false;
        }),
        debounceTime(750),
        distinctUntilChanged(),
        switchMap(term =>
          this.cityService.getLocation(term).pipe(
            tap((response) => {
              this.searchFailed = response.length === 0;
              this.isSearching = false;
            })))
      );
  }

spec.ts

  it('should call spy on city search', fakeAsync(() => {
    component.user = <User>{uid: 'test', username: 'mleko', location: null, description: null};
    const spy = (<jasmine.Spy>cityStub.getLocation).and.returnValue(of['München Bayern']);

    fixture.detectChanges();
    const compiled: DebugElement = fixture.debugElement.query(By.css('#locationEdit'));
    compiled.nativeElement.value = 'München';
    compiled.nativeElement.dispatchEvent(new Event('input'));

    tick(1000);
    fixture.detectChanges();

    expect(spy).toHaveBeenCalled();
  }));

Can someone help me to mock this.search propperly?

from Angular testing - ngBootstraps typeahead