Invoice Id generation needs an extension mechanism

A customised invoice_id cannot be generated in an extension. A more extensive description can is found at http://civicrm.stackexchange.com/questions/16871/how-do-i-generate-a-custom-invoice-id .

The update is done in CRM_Contribute_Form_Task_Invoice on line 546 with the following line:

CRM_Core_DAO::setFieldValue('CRM_Contribute_DAO_Contribution', $contribution->id, 'invoice_id', $invoiceId);

Remark:
At the moment the invoice is derived from the contribution id. So it does not matter if the invoice code is recalculated or red directly from the database. But if a different calculation is used a new invoice id is generated every time a invoice is printed.