title: TypeScript: Firestore withConverter and refrences
body: <p>I am fairly new to Firestore/Firebase. I have this Album class that can hold a reference to a parent Album</p>
  
  <pre><code>export class Album {
      constructor(public id: string,
                  public title: string,
                  public parent: Album | null,
      ) {
      }
  }
  
  export class AlbumConverter implements FirestoreDataConverter&lt;ImageData&gt; {
      toFirestore(album: Album): firebase.firestore.DocumentData {
          return {
              id: album.id,
              title: album.title,
              parent: album.parent ?.....
  </code></pre>
  
  <p>My question is, how should I implement the <code>toFirestore</code> and <code>fromFirestore</code>? what's the proper way to use <code>DataConverter</code> when dealing with references?</p>
  
  <p>Thank you!</p>