def __init__(self, course, *args, **kwargs):
        super(RegistrationForm, self).__init__(*args, **kwargs)

        self.fields['items'].queryset = BarterItem.objects.filter(
            course=course)
        self.fields['items'].error_messages['required'] = _(
            "Please select at least one item")
        self.fields['items'].empty_label = None