Many2Many comma separated form field

from django.forms.models import ModelMultipleChoiceField
from django.forms.widgets import TextInput
from django.forms.util import ValidationError
from django.utils.encoding import force_unicode

class ModelMultipleCommaField(ModelMultipleChoiceField):
    widget = TextInput
    
    def clean(self,value):
        if self.required and not value:
            raise ValidationError(self.error_messages['required'])
        elif not self.required and not value:
            return []
        
        value=value.split(',')
        
        if not isinstance(value, (list, tuple)):
            raise ValidationError(self.error_messages['list'])
        
        for pk in value:
            try:
                self.queryset.filter(pk=pk)
            except ValueError:
                raise ValidationError(self.error_messages['invalid_pk_value'] % pk)
        qs = self.queryset.filter(pk__in=value)
        pks = set([force_unicode(o.pk) for o in qs])
        
        for val in value:
            if force_unicode(val) not in pks:
                raise ValidationError(self.error_messages['invalid_choice'] % val)
        return qs