angular - Using async pipe with ngFor

Question

Ultimate goal is to use nested ngFor's created dynamically. I try to create a series of drop-down menus, each depending on the previous one. The exact number of drop-down menus is unknown and created dynamically. Example:

<form [ngFormModel]="dropDownForm" (ngSubmit)="onSubmit()">
    <div *ngFor="#nr of numberOfDropdowns">
      <label>{{nr.name}}</label>
      <select [ngFormControl]="dropDownForm.controls[i]">
          <option  *ngFor="#item of Dropdown[nr.id] | async" value="{{item.value}}">{{item.name}}</option>
      </select>
    </div>
  <button type="submit">Submit</button>
</form>

The whole things fails at Dropdown[nr.id] which does not seem to work with async pipe. I played around a bit:

{{myAsyncObject | async}} //works
{{myAsyncObject['prop1'] | async}} //fails silently
{{myAsyncObject['prop1']['prop2'] | async}} // EXCEPTION: TypeError: Cannot read property 'prop2' of undefined in [null]    

Any ideas on how to get this working?

Answer 1

Just want to add an alternative that worked for me (no extra pipe necessary):

*ngFor="#obj of (myAsyncObject | async)?.prop1?.prop2"