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.
They are to complete the invoice, so it can be in an invalid date, but
we do not want to force people to finish all required inputs before they
can add products or update quantities, do we?
Now had to add the empty option label for customer in all cases, because
it could be empty, although that should be done regardless in case
someone has a browser that does not validate fields.
jordi fita mas