title: How to specify child objects type in an NSArray with Mantle
body: <p>If I have a dictionary like</p>
  
  <pre><code>{
    name: "Bob",
    cars: [
      { make: "ford", year: "1972" },
      { make: "mazda", year: "2000" }
    ],
  }
  </code></pre>
  
  <p>and two models like:</p>
  
  <pre><code>@interface CarModel : MTLModel
  
  @property (nonatomic, strong) NSString *make;
  @property (nonatomic, strong) NSString *year;
  
  @end
  
  @interface PersonModel : MTLModel
  
  @property (nonatomic, strong) NSString *name;
  @property (nonatomic, strong) NSArray *cars;
  
  @end
  </code></pre>
  
  <p>How do I use Mantle so that my array of cars in my person model are CarModels?</p>