We are going to allow invoices with products that are not (yet) inserted
into the products table.
We always allowed to have products in invoices with a totally different
name, description, price, and whatnot, but until now we had the product
id in these invoice lines for statistics purposes.
However, Oriol raised the concern that this requires for the products
to be inserted before we can create an invoice with them, and we do not
plan to have a “create product while invoicing” feature, thus it would
mean that people would need to cancel the new invoice, create the new
product, and then start the invoice again from scratch.
The compromise is to allow products in the invoice that do not have a
product_id, meaning that at the time the invoice was created they were
not (yet) in the products table. Oriol sees this stop-invoice-create-
product issue more important than the accurate statistics of product
sales, as it will probably be only one or two units off, anyway.
I did not want to allow NULL values to the invoice product’s product_id
field, because NULL means “dunno” instead of “no product”, so i had to
split that field to a separate table that relates an invoice product
with a registered product.
I had to use a deferrable foreign key because the payment methods have
a reference to the company, and the company now a circular reference to
payment method.
We want to show the percentage of the tax as columns in the invoice,
but until now it was not possible to have a single VAT column when
products have different VAT (e.g., 4 % and 10 %), because, as far
as the application is concerned, these where ”different taxes”. We
also think it would be hard later on to compute the tax due to the
government.
So, tax classes is just a taxonomy to be able to have different names
and rates for the same type of tax, mostly VAT and retention in our
case.
They are not functions because i need to join them with the main
invoice relation, and although possible is a bit more awkward with
functions.
The taxes have their own relation because i will need them grouped by
their name in the PDF, so it will probably be a select for that
relation.
jordi fita mas