title: django rest framework ModelSerializer post data error
body: <p>first, in <strong>models.py</strong>  </p>
  
  <pre><code>class UserComment(models.Model):
      user = models.ForeignKey(User)
      rate = models.IntegerField()
      description = models.CharField(max_length=512)
      createTime = models.DateTimeField(auto_now=True)
      def __unicode__(self):
          return '&lt;UserComment {%s %d}&gt;' % (self.user.username, self.rate)
  </code></pre>
  
  <p>then, <strong>serializers.py</strong></p>
  
  <pre><code>class UserSerializer(serializers.ModelSerializer):
      class Meta:
          model = User
          fields = ('id', 'username', 'email', )
  
  class UserCommentSerializer(serializers.ModelSerializer):
      user = UserSerializer(required=False)
      class Meta:
          model = UserComment
  </code></pre>
  
  <p><strong>views.py</strong></p>
  
  <pre><code>class UserCommentViewSet(viewsets.ModelViewSet):
      queryset = UserComment.objects.all()
      serializer_class = UserCommentSerializer
      permission_classes = (permissions.IsAuthenticatedOrReadOnly, )
  
      def create(self, request, *args, **kwargs):
          serializer = self.get_serializer(data=request.DATA, files=request.FILES)
          serializer.is_valid()
          print serializer.errors
          print serializer.data
          return super(UserCommentViewSet, self).create(request, *args, **kwargs)
  </code></pre>
  
  <p>then i post json data 
  <code>{"user":{"id":"1","username":"watsy"},"rate":"5","description":"hello"}</code></p>
  
  <p>i think,it will work. and insert it to db, but i get errors.</p>
  
  <p><code>{"user": [{"username": ["User with this Username already exists."]}]}</code><br>
  <strong>>_&lt;</strong>, I have no idea.</p>