invoice preferences collision

I tried to use different invoice series numbering( for example int001 , int002 etc for local invoices, ext 001, ext002 etc for external (international) invoices. But since in si_invoices table we have the preference_id field, in si_invoice_items this is absent so if the invoice_id is the same all items will be added to all invoices. I think it is not possible even to create an estimate because of this issue. So IMHO the preference_id field should be added to si_invoice_items too and used in all related queries.