Agreed. Invoices, for example, are a representation of history so they need the exact data that was used at the time. They can have a reference to a product id but at any time it's possible for the data associated with that product to be completely changed from the meaning at the time of the invoice. This happens with, e.g., UPCs, which can be reused. On the other hand, if you have assemblies of products then you probably want those items to update themselves automatically.