code: def save(self):
          
          if not self.vads_trans_date:
              self.vads_trans_date = datetime.datetime.utcnow().replace(
                  tzinfo=utc).strftime("%Y%m%d%H%M%S")
          if not self.vads_trans_id:
              self.vads_trans_id = tools.get_vads_trans_id(
                  self.vads_site_id, self.vads_trans_date)
          if self.theme and not self.vads_theme_config:
              self.vads_theme_config = str(self.theme)
          if not self.pk:
              super(PaymentRequest, self).save()
          self.set_vads_payment_config()
          self.set_signature()
          super(PaymentRequest, self).save()
docs: We set up vads_trans_id and theme according to payzen format.
  
          If fields values are explicitely set by user, we do not override
          their values.