django-rest-framework tutorial 4 (authentic​ation-and-​permission​s) issue

After finishing tutorial 1-3 smoothly, I followed the tutorial (http://django-rest-framework.org/tutorial/4-authentication-and-permissions.html) and finished sections up to "Adding endpoints for our User models".

(In other words, adding "url(r'^users/$', views.UserList.as_view()), url(r'^users/(?P[0-9]+)/$', views.UserInstance.as_view())," was done.)

Then, I ran the server with the command of "python manage.py runserver" and point my browser to http://127.0.0.1:8000/users/ and got the following error message:

(message starts)

NameError at /users/

name 'User' is not defined

Request Method: GET

Request URL: http://127.0.0.1:8000/users/

Django Version: 1.4.3

Exception Type: NameError

Exception Value: name 'User' is not defined

Exception Location: /home/user/tutorial/snippets/serializers.py in Meta, line 14

(message ends)

Did I miss something?

The code in my serializer.py was:

from django.forms import widgets
from rest_framework import serializers
from snippets import models

class SnippetSerializer(serializers.ModelSerializer):
    class Meta:
        model = models.Snippet
        fields = ('id', 'title', 'code', 'linenos', 'language', 'style')

class UserSerializer(serializers.ModelSerializer):
    snippets = serializers.ManyPrimaryKeyRelatedField()

    class Meta:
        model = User
        fields = ('id', 'username', 'snippets')