def unpack_question_dimid(dimension_id):
    """Decompose the dimension id into unit, lesson and question id.

    Returns:
        A tuple unit_id, lesson_id, question_id.
        unit_id and question_id are strings. lesson_id can be a string or
        None.
    """
    unit_id, lesson_id, question_id = dimension_id.split(':')
    if lesson_id == 'None':
        lesson_id = None
    return unit_id, lesson_id, question_id