title: How to get FormArrayName when the FormArray is nested in another FormArray?
body: <p>Refering to : <a href="https://angular.io/docs/ts/latest/api/forms/index/FormArrayName-directive.html" rel="noreferrer">https://angular.io/docs/ts/latest/api/forms/index/FormArrayName-directive.html</a>, it is easy to get a FormArrayName :</p>
  <pre class="lang-html prettyprint-override"><code>&lt;form [formGroup]=&quot;form&quot; (ngSubmit)=&quot;onSubmit()&quot;&gt;
    &lt;div formArrayName=&quot;cities&quot;&gt;
      &lt;div *ngFor=&quot;let city of cities.controls; index as i&quot;&gt;
         &lt;input [formControlName]=&quot;i&quot; placeholder=&quot;City&quot;&gt;
      &lt;/div&gt;
    &lt;/div&gt;
    &lt;button&gt;Submit&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  <pre class="lang-ts prettyprint-override"><code>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!
  </code></pre>
  <p>The DOM <code>&lt;div formArrayName=&quot;cities&quot;&gt;</code>
  uses the getter
  <code>get cities(): FormArray { return this.form.get('cities') as FormArray; } </code>
  and everything is working like a charm</p>
  <h2>BUT</h2>
  <p>How to make the getter when the FormArray is nested in another FormArray?</p>
  <p>Let's say this example :</p>
  <pre class="lang-ts prettyprint-override"><code>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.
  </code></pre>
  <pre class="lang-html prettyprint-override"><code>&lt;form [formGroup]=&quot;form&quot; (ngSubmit)=&quot;onSubmit()&quot;&gt;
    &lt;div formArrayName=&quot;cities&quot;&gt;
      &lt;div *ngFor=&quot;let city of cities.controls; index as i&quot; [formGroupName]=&quot;i&quot;&gt;
         &lt;input [formControlName]=&quot;name&quot; placeholder=&quot;City&quot;&gt;
         &lt;div formArrayName=&quot;sisterCities&quot;&gt; &lt;!-- this will never work --&gt;
           &lt;div *ngFor=&quot;let sisterCity of sisteCities.controls; index as j&quot;&gt;
             ...
           &lt;/div&gt;
         &lt;/div&gt;
      &lt;/div&gt;
    &lt;/div&gt;
    &lt;button&gt;Submit&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  <p>Please, help me to achieve this.
  Thank you in advance.</p>