Referindo-se a:https://angular.io/docs/ts/latest/api/forms/index/FormArrayName-directive.html, é fácil obter um FormArrayName:

HTML:

<form [formGroup]="form" (ngSubmit)="onSubmit()">
  <div formArrayName="cities">
    <div *ngFor="let city of cities.controls; index as i">
       <input [formControlName]="i" placeholder="City">
    </div>
  </div>
  <button>Submit</button>
</form>

TS:

form = new FormGroup({
  cities: new FormArray([
    new FormControl('SF'),
    new FormControl('NY'),
  ]),
});

get cities(): FormArray { return this.form.get('cities') as FormArray; } 
// This does the magic!

O DOM<div formArrayName="cities"> usa o getterget cities(): FormArray { return this.form.get('cities') as FormArray; } e tudo está funcionando como um encanto

MAS

Como fazer o getter quando o FormArray está aninhado em outro FormArray?

Digamos este exemplo:

TS:

form = new FormGroup({
  cities: new FormArray([
    new FormGroup({ 
      name: new FormControl('SF'),
      sisterCities: new FormArray(['Shanghai','Zurich',...])
    }),
    new FormGroup({
      name: new FormControl('NY'),
      sisterCities: new FormArray(['London','Oslo',...])
    }),
  ]),
});

get cities(): FormArray { return this.form.get('cities') as FormArray; } 
// still get the main cities FormArray

// but 
// get sisterCities() won't work because I need to target a city FormGroup (NY or SF) before accessing its sisterCities FormArray.
// and AFAIK, it is not possible to pass parameters to a getter.


HTML:

<form [formGroup]="form" (ngSubmit)="onSubmit()">
  <div formArrayName="cities">
    <div *ngFor="let city of cities.controls; index as i" [formGroupName]="i">
       <input [formControlName]="name" placeholder="City">
       <div formArrayName="sisterCities"> <!-- this will never work -->
         <div *ngFor="let sisterCity of sisteCities.controls; index as j">
           ...
         </div>
       </div>
    </div>
  </div>
  <button>Submit</button>
</form>


Please, help me to achieve this.
Thank you in advance.