models.py

    def react_to_purchasing_status(self):
        if self.purchasing_status >= PURCHASE_STATUS_SUBMITTED and self.submitted_at is None:
            self.submitted_at = datetime.now()

            for line in self.purchaseline_set.filter(unit_cost=None):
                line.unit_cost = line.promotional_cost_at_submit_time
                line.save()
        else:
            self.submitted_at = None
        # Then the rest