I'd like to know if there is a restriction using inheritance about multiplejoin function because I'm getting an error and I don't know why. I've the following code:

class Question(InheritableSQLObject):
    filename = StringCol(default=None, length=255, unique=True)
    title = StringCol(default=None)
    default_mark = FloatCol(default=0.0)
    wording = RelatedJoin('Content')
    term_questions = RelatedJoin('Term', joinColumn='term', otherColumn='question',
    answers = MultipleJoin('Answer')
    questionref = MultipleJoin('QuestionRef')

class Answer(SQLObject):
    value = StringCol(default=None)
    calification = FloatCol(default=None)
    ide = StringCol(default=None)
    question = ForeignKey('Question')

class Ord(Question):
    _inheritable = False
    shuffle = BoolCol()
    orderanswers = MultipleJoin('OrderChoice')

class OrderChoice(SQLObject):
    value = StringCol(default=None)
    ord = IntCol(default=None)
    ide = StringCol(default=None)
    question = ForeignKey('Ord')

and when I've a Ord instance (called 'q') and I do 'q.orderanswers' I get the following error:
sqlobject.dberrors.OperationalError: Unknown column 'ord_id' in 'where clause'

Any idea?