Does belongsTo create an instance of the corresponding class?

Question

I have the following:

class Book
{
  static belongsTo = [cart: Cart]
}

When I instantiate Book, does it automatically create an instance of Cart and then attach book to it?

Answer 1

This creates a field in the class of type Cart with name cart, as if you had this in your source code:

class Book {
   Cart cart
   static belongsTo = [cart: Cart]
}

It's a persistent property, like String title, etc. But since its type is another domain class it's loaded lazily by default (you can configure this in the mapping or per-query). So loading a Book instance doesn't load its owning Cart instance, but when you reference one of the cart field's properties, it will lazily load.

Additionally, since you've declared a belongs-to relationship, when you delete a Cart, all of its Books will be deleted too.