I store again the product’s name, description, and prices, because they
are bound to change, but the invoice should remain the same always.
That makes me wonder if i should do the same for seller’s and buyer’s
data, but that should be a different commit.
I’ve added the discount_rate domain because then i can test it
independently of the invoice_product relation, moreover i am sure i will
need it for simplified invoices too.
jordi fita mas
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