What's the best way to support aggregate data structures using content types? For example, if I have an order processing system, then I might have data types like Order and OrderLineItem, where each Order may have reference to none or more OrderLineItems. How do I model this relationship with content types?
Do I base the Order content type on the built-in Folder type? Do I have separate content types where an OrderLineItem content type would contain a lookup field of an Order, and then heavily customized forms to render this on the Orders list?