I am trying to adjust the workflow of an invoice... that is a custom user (in my case, an equivalent of Finance Administrator) must approve before it is paid.

How do i differentiate an invoice for 'sale' and one for 'purchase' ? That is, a supplier payment out must be validated...but not a customer payment (receivable). Here's a snippet:

if not self.pool['res.users'].has_group(cr, uid, 'purchase_requisition.cdn_proculator'):
                raise openerp.exceptions.AccessError(_("Only CDN Proculator can approve."))

tried:

if not self.pool['res.users'].has_group(cr, uid, 'purchase_requisition.cdn_proculator') and invoice.type == 'purchase':
                raise openerp.exceptions.AccessError(_("Only CDN Proculator can approve."))

but didn't work. How can i differentiate the account.invoice ?