This is for new users that do not start using the application from the
beginning of the current fiscal year and, therefore, need to create
invoices starting from a specific number.
I had to change the constraint on the currval to allow zero, otherwise
it would not be possible to set 1 as the next number, because users
can also not delete the row.
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.
I can not use a PostgreSQL sequence because invoices need to be gapless,
and sequences are designed to not rollback, for performance reasons. In
this case, the performance is secondary because the law does not care.
jordi fita mas