title: How can the check printing report be modified to include a supplier's invoice number?
body: <p>When I receive an invoice from a supplier, I enter the invoice number (from the supplier) in the "Source Document" field of the supplier invoice. Now, when I print checks (using account_check_writing) it includes the supplier invoice number that OpenERP generates for my record of the supplier's invoice (for example, EXJ/2012/01283), but nowhere does it include the supplier's invoice number. This makes it difficult for the supplier to find the correct invoice in their records to apply the payment.</p>
  
  <p>I have a handle on how to modify reports generally, but I'm not sure of the expression that will evaluate to the supplier's invoice number. I can follow the foreign key constraints from the database to see that account_voucher_line -> account_move_line -> account_move &lt;- account_invoice, and I can find the account.move associated with the current voucher line in the report with "l.move_line_id.move_id". But now how do I find the invoice that also references that move_id, so I can grab the "Source Document" from it?</p>