[jdmichal]

I don't understand. That's exactly what domain modeling is. Putting names to the data you need to store and, in this case, capture at a particular moment in time.

Let's put it this way. Do you think it is reasonable to store the taxes as a reference, or the shipping cost? We would never consider it appropriate to say, "Go look up the shipping cost for a 5lb package from here to here." Oh, and by the way, this is using a live reference to a customer object, for which the shipping address might very well have changed.

Of course we would not do this. So why would it appropriate for a product price? An invoice is the capture of all this information at the moment the purchase is made. That is its purpose. If we did not need this moment-in-time capture, then the concept of an invoice would not even exist.