When you define a report, you have to associate it to a model. When the report is saved, the ir.attachment record will hold the model and a record id which, typically, is the active record from which you called the report. Great.

...But, what about reports called from wizards? Wizards have temporary id's so... if the report has a model associated, the attachment record will be associated to that model and the wizard's temporary id! This will cause the attachment to be related to a random record!

How can I tell Odoo/OpenERP that I want the attachment record to be related to a specific id (for instance, a value coming from one of the wizard's fields)?