A ToscaWidgets2 widget that needs a dynamic SelectField and does not work :(

submission.py

# -*- coding: utf-8 -*-
'''
Created on 17.03.2012

@author: moschlar
'''

import tw2.core as twc
import tw2.forms as twf
import tw2.sqla as twsa
import tw2.bootstrap as twb

from sauce.model import Language, Assignment


class DbSelectionField(twf.SelectionField):
    entity = twc.Param('SQLAlchemy mapped class to use', request_local=False)
    query = twc.Param(request_local=False)

    def prepare(self):
        self.options = [(x.id, unicode(x)) for x in self.query.all()]
        super(DbSelectionField, self).prepare()


class LanguageSelectField(twb.SingleSelectField, DbSelectionField):
    @classmethod
    def post_define(cls):
        if getattr(cls, 'entity', None):
            cls.validator = twsa.RelatedValidator(entity=cls.entity)


class SubmissionForm(twb.HorizontalForm):

    assignment_id = twf.HiddenField()
    submission_id = twf.HiddenField()

    filename = twb.TextField()
    source = twb.TextArea()
    source_file = twb.FileField()

    language_id = LanguageSelectField(entity=Language)

    def prepare(self):
        #self.language_id.query = Language.query.filter(
        self.child.c.language_id.query = Language.query.filter(
            Language.id.in_(Assignment.query.get(self.value.assignment_id).allowed_languages))
        super(SubmissionForm, self).prepare()