Add role to user
        Role must be valid and saved first, otherwise will
        raise an exception.

def add_role(self, role):
        """
        Add role to user
        Role must be valid and saved first, otherwise will
        raise an exception.
        """
        schema = RoleSchema()
        ok = schema.process(role)
        if not ok or not role.id:
            err = 'Role must be valid and saved before adding to user'
            raise x.UserException(err)

        self.__roles.append(role)