gistfile1.txt

* Invoices table includes an invoice number.
* InvoiceNumbers table is a singleton and contains the 'next' invoice number.
* To generate a new invoice, in a transaction:
  - Increment the InvoiceNumbers singleton to get your next number
  - Create the invoice and save it with the number contained therein
* Any request to find an invoice number which is not recorded must map to a case where
  somehow the InvoiceNumbers increment succeeded but the invoice was not generated and
  despite this transaction failure, nothing rolled back. This should never happen, but
  if it does, you can state logically that there is no real gap for audit purposes.
* We assume that VAT invoices are not generated very frequently (they're generated by
  specific customer request) and use a table-level database (pessimistic) lock in
  Rails to ensure that all InvoiceNumber increments are atomic