Django MultiSelectField

fields.py

"""
A model field and widget that allows selecting multiple items from a static 
list of choices. Based on http://djangosnippets.org/snippets/1200/
"""
from django.db import models
from django.core import exceptions
from django import forms
from django.utils.text import capfirst

class MultiSelectFormField(forms.MultipleChoiceField):
    widget = forms.CheckboxSelectMultiple
    
    def __init__(self, *args, **kwargs):
        self.max_choices = kwargs.pop('max_choices', 0)
        super(MultiSelectFormField, self).__init__(*args, **kwargs)

    def clean(self, value):
        if not value and self.required:
            raise forms.ValidationError(self.error_messages['required'])
        if value and self.max_choices and len(value) > self.max_choices:
            raise forms.ValidationError('You must select a maximum of %s choice%s.'
                    % (apnumber(self.max_choices), pluralize(self.max_choices)))
        return value
    
    def validate(self, value, model_instance):
        arr_choices = self.get_choices_selected(self.get_choices_default())
        for opt_select in value:
            if (opt_select not in arr_choices): 
                raise exceptions.ValidationError(self.error_messages['invalid_choice'] % value)    
        return
    
    def get_choices_selected(self, arr_choices=''):
        if not arr_choices:
            return False
        list = []
        for choice_selected in arr_choices:
            list.append(choice_selected[0])
        return list


class MultiSelectField(models.Field):
    __metaclass__ = models.SubfieldBase

    def get_internal_type(self):
        return "CharField"

    def get_choices_default(self):
        return self.get_choices(include_blank=False)

    def _get_FIELD_display(self, field):
        value = getattr(self, field.attname)
        choicedict = dict(field.choices)
    
    def value_to_string(self, obj):
        value = self._get_val_from_obj(obj)
        return self.get_db_prep_value(value)
    
    def formfield(self, **kwargs):
        # don't call super, as that overrides default widget if it has choices
        defaults = {'required': not self.blank, 'label': capfirst(self.verbose_name), 
                    'help_text': self.help_text, 'choices':self.choices}
        if self.has_default():
            defaults['initial'] = self.get_default()
        defaults.update(kwargs)
        return MultiSelectFormField(**defaults)

    def get_db_prep_value(self, value, *args, **kwargs):
        if isinstance(value, basestring):
            return value
        elif isinstance(value, list):
            return ",".join(value)
    
    def to_python(self, value):
        if isinstance(value, (list, tuple)):
            return value
        elif value is None:
            return ''
        return value.split(",")
    
    def contribute_to_class(self, cls, name):
        super(MultiSelectField, self).contribute_to_class(cls, name)
        if self.choices:
            func = lambda self, fieldname = name, choicedict = dict(self.choices):",".join([choicedict.get(value,value) for value in getattr(self,fieldname)])
            setattr(cls, 'get_%s_display' % self.name, func)

Not work on Django 1.3.1
"Value ['a', 'b', 'c'] is not a valid choice."

I am don't know how it is fix.