Gets the grade ``Ids`` in this system ranked from highest to lowest.

        return: (osid.id.IdList) - the list of grades ``Ids``
        raise:  IllegalState - ``is_based_on_grades()`` is ``false``
        *compliance: mandatory -- This method must be implemented.*

def get_grade_ids(self):
        """Gets the grade ``Ids`` in this system ranked from highest to lowest.

        return: (osid.id.IdList) - the list of grades ``Ids``
        raise:  IllegalState - ``is_based_on_grades()`` is ``false``
        *compliance: mandatory -- This method must be implemented.*

        """
        # Implemented from template for osid.repository.Asset.get_asset_content_ids_template
        id_list = []
        for grade in self.get_grades():
            id_list.append(grade.get_id())
        return IdList(id_list)