PartitionKey mapper option

[Mararok]

Can you add mapper option to PartitionKey decorator ?

Now I must hack it with other field

export class UserDynamoDb implements User {
  @PartitionKey()
  public _id: string;

  @Transient()
  public id: UserId; // after 'store.get(...)' I must setup it manually 

  @Property()
  public username?: string;
}

It will be very helpful for me :)