NameError：name＆＃39; ModelSelect2MultipleField＆＃39;没有定义

Question

当我运行一个说明ModelSelect2MultipleField未定义的程序时，我收到错误。我在这里用它：

from django import forms
from django_select2 import *
from django.db.models import Q
from contacts.models import Contact
from cards.models import JobCard
from employee.models import *

from django.core.exceptions import ValidationError

def validate_fail_always(value):
    raise ValidationError(u'%s not valid. Infact nothing is valid!' % value)

########### Forms ##############]

class GetContactSearchForm(forms.Form):
    clients = ModelSelect2MultipleField(queryset=Contact.objects, required=False)

class GetEmployeeSearchForm(forms.Form):
    employees = ModelSelect2MultipleField(queryset=Employee.objects, required=False)

我的python版本为2.7.10，我的django为1.8;我应该用我的settings.py文件替换那个字段，或者我需要安装或放入其他东西吗？ 谢谢！