I am working on a small project with some peculiar issues.
I have to track SUPPLIERS, SHIPMENTS, PRODUCTS, AND SALE. Now these are the major entities.
The problems is this:
There is a concept ITEM which is required as product seems to be just a category (Name, type, properties). As I need to track each individual ITEM not just reduced quantity in stock for a PRODUCT sold. As an ITEM might be returned defective, therefore requiring them to be tracked seperatly.
Now these are the relationships and cardinalities:
SUPPLIER ---sends--- SHIPMENT (1:N )
ITEM -----make-----PRODUCT (M:N) ** has we can combine same items to form different products. eg. Item01 - biscuits can be packaged with item03 coffee. Or they can be sold seperately as products individually.
THE ISSUE: HOW DOES SALE fit in ? Items are sold or PRODUCTS are sold. Items are put on offer (like auction listing for example) OR PRODUCTS are listed ?.
It is true that some kinds of product are tracked individually (like PCs, TVs, cars - they have serial numbers to identify them), and some are not (like biscuits, coffee, spanners, newspapers, ...) You'd have to treat the former differently from the latter.
However, your post seems to imply that you intend to track individual packets of biscuits or jars of coffee! Nobody does that surely!?
Anyway, for items that are tracked individually, I imagine you would start off with an order that related to a product, not to a specific item: "customer X has ordered 1 of product Y". Then at some point in the process (maybe days or weeks later) you will assign as specific item to the order: "item Z assigned to order O".