vads_payment_config can be set only after object saving.

        A custom payment config can be set once PaymentRequest saved
        (adding elements to the m2m relationship). As a consequence
        we set vads_payment_config just before sending data elements
        to payzen.

def set_vads_payment_config(self):
        """
        vads_payment_config can be set only after object saving.

        A custom payment config can be set once PaymentRequest saved
        (adding elements to the m2m relationship). As a consequence
        we set vads_payment_config just before sending data elements
        to payzen."""
        self.vads_payment_config = tools.get_vads_payment_config(
            self.payment_config, self.custom_payment_config.all())