def METHOD_NAME(self):
    for mandate in self:
        if mandate.state == "valid":
            if not mandate.signature_date:
                raise ValidationError(
                    _(
                        "Cannot validate the mandate '%s' without a date of "
                        "signature."
                    )
                    % mandate.unique_mandate_reference
                )
            if not mandate.partner_bank_id:
                raise ValidationError(
                    _(
                        "Cannot validate the mandate '%s' because it is not "
                        "attached to a bank account."
                    )
                    % mandate.unique_mandate_reference
                )