CoreData: multiple copies of the same item linked to another?

Question

In Core Data is there a way to have one instance of an object to have multiple links to another? This is best understood with an example. You have a shopping cart object, ShoppingCart, and you have books, Book. How do you properly put multiple copies of the same book into the cart?

No matter how many times you run [shoppingCart addBooksObject:book]; it will only show up once.

Right now I have a many-to-many connection between the two, but since shoppingCart.books is a set, it removes the duplicates. How do I get around that?

Answer 1

You get around it by using an appropriate data model. :-)

This is the classic "line item" problem. There's Product, Invoice, and Line Item. In your case, the book is the product and the shopping cart the invoice. You don't put a product in the cart, you put a line item (that is linked to the product) in the cart.

The line item(s) belong to the cart (one per product) and hold the quantity attribute (and maybe a computed subtotal based on some volume discount based on quantity). In other words, you need an intermediary entity (maybe CartItem?) to hold the relationship and the quantity.