title: 'ListSerializer' object is not callable
body: <p>I am trying to implement Django-Rest framework for a voting application with content_type objects.  I tried using the <a href="https://github.com/Ian-Foote/rest-framework-generic-relations" rel="noreferrer">rest-framework-generic-relations</a> for serializers.py.  It seems to me that the error might be within serializer.py / views.py, but I am new to this framework and would appreciate your help!</p>
  
  <p>views.py:</p>
  
  <pre><code>class vote_detail(generics.RetrieveUpdateDestroyAPIView):
      queryset = VotedItem.objects.all()
      serializer_class = VoteSerializer(queryset, many=True)
  </code></pre>
  
  <p>serializers.py:</p>
  
  <pre><code>from rest_framework import serializers
  from .models import VotedItem
  from posts.models import Post
  from generic_relations.relations import GenericRelatedField
  
  class PostSerializer(serializers.ModelSerializer):
      class Meta:
          model = Post
          fields = ('title',)
  
  class VoteSerializer(serializers.ModelSerializer):
          """
          A `VotedItem` serializer with a `GenericRelatedField` mapping all possible
          models to their respective serializers.
          """
      voted_object = GenericRelatedField({
          Post: PostSerializer(),
      })
  
      class Meta:
          model = VotedItem
          fields = ('user', 'voted_object')
  </code></pre>
  
  <p>and models.py:</p>
  
  <pre><code>from __future__ import unicode_literals
  
  from datetime import datetime
  
  from django.utils.encoding import python_2_unicode_compatible
  from django.db import models
  from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
  from django.contrib.contenttypes.models import ContentType
  from django.core.urlresolvers import reverse
  from django.utils import timezone
  from django.contrib.auth.models import User
  from social_count.managers import VoteManager
  
  SCORES = (
      (+1, '+1'),
      (-1, '-1'),
  )
  
  @python_2_unicode_compatible
  class VotedItem(models.Model):
      """
      A vote on an object by a User.
      """
      user = models.ForeignKey(User)
      content_type = models.ForeignKey(
          ContentType,
          null=True,
          blank=True,
          on_delete=models.CASCADE,
      )
      object_id = models.PositiveIntegerField(        
          null=True,
      )
      content_object = GenericForeignKey('content_type', 'object_id')
      vote = models.SmallIntegerField(choices=SCORES, blank=True, null=True)
      flag = models.NullBooleanField(blank=True, null=True)
      timestamp = models.DateTimeField(auto_now=False, auto_now_add=True)
  
      objects = VoteManager()
  
      class Meta:
          db_table = 'votes'
          # One vote per user per object
          unique_together = (('user', 'content_type', 'object_id'),)
  
      def __str__(self):
          return '%s: %s on %s' % (self.user, self.vote, self.content_object)
  
      def is_upvote(self):
          return self.vote == 1
  
      def is_downvote(self):
          return self.vote == -1
  </code></pre>
  
  <p>When I visit www.mypage.com/likes/api/1/ I get an error message:</p>
  
  <pre><code>Traceback:
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
    149.                     response = self.process_exception_by_middleware(e, request)
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
    147.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/django/views/decorators/csrf.py" in wrapped_view
    58.         return view_func(*args, **kwargs)
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/django/views/generic/base.py" in view
    68.             return self.dispatch(request, *args, **kwargs)
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/rest_framework/views.py" in dispatch
    466.             response = self.handle_exception(exc)
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/rest_framework/views.py" in dispatch
    463.             response = handler(request, *args, **kwargs)
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/rest_framework/generics.py" in get
    286.         return self.retrieve(request, *args, **kwargs)
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/rest_framework/mixins.py" in retrieve
    57.         serializer = self.get_serializer(instance)
  
  File "/home/henry/newpine/src/env/local/lib/python2.7/site-packages/rest_framework/generics.py" in get_serializer
    111.         return serializer_class(*args, **kwargs)
  
  Exception Type: TypeError at /likes/api/1/
  Exception Value: 'ListSerializer' object is not callable
  </code></pre>