:returns: the ID of the assertion in the response
        :rtype: string

def get_assertion_id(self):
        """
        :returns: the ID of the assertion in the response
        :rtype: string
        """
        if not self.validate_num_assertions():
            raise OneLogin_Saml2_ValidationError(
                'SAML Response must contain 1 assertion',
                OneLogin_Saml2_ValidationError.WRONG_NUMBER_OF_ASSERTIONS
            )
        return self.__query_assertion('')[0].get('ID', None)