title: AttributeError 'tuple' object has no attribute 'values' django rest framework
body: <p>i am trying to get data from django server and get this error. </p>
  
  <pre><code>Internal Server Error: /data/site-info/
  Traceback (most recent call last):
    File "/Library/Python/2.7/site-packages/django/core/handlers/base.py", line 149, in get_response
      response = self.process_exception_by_middleware(e, request)
    File "/Library/Python/2.7/site-packages/django/core/handlers/base.py", line 147, in get_response
      response = wrapped_callback(request, *callback_args, **callback_kwargs)
    File "/Library/Python/2.7/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
      return view_func(*args, **kwargs)
    File "/Library/Python/2.7/site-packages/django/views/generic/base.py", line 68, in view
      return self.dispatch(request, *args, **kwargs)
    File "/Library/Python/2.7/site-packages/rest_framework/views.py", line 466, in dispatch
      response = self.handle_exception(exc)
    File "/Library/Python/2.7/site-packages/rest_framework/views.py", line 463, in dispatch
      response = handler(request, *args, **kwargs)
    File "/Library/Python/2.7/site-packages/rest_framework/decorators.py", line 53, in handler
      return func(*args, **kwargs)
    File "/Users/hco/PycharmProjects/tool/linyit/data/views.py", line 17, in site_info
      Response(serializer.data)
    File "/Library/Python/2.7/site-packages/rest_framework/serializers.py", line 674, in data
      ret = super(ListSerializer, self).data
    File "/Library/Python/2.7/site-packages/rest_framework/serializers.py", line 239, in data
      self._data = self.to_representation(self.instance)
    File "/Library/Python/2.7/site-packages/rest_framework/serializers.py", line 614, in to_representation
      self.child.to_representation(item) for item in iterable
    File "/Library/Python/2.7/site-packages/rest_framework/serializers.py", line 459, in to_representation
      fields = self._readable_fields
    File "/Library/Python/2.7/site-packages/django/utils/functional.py", line 33, in __get__
      res = instance.__dict__[self.name] = self.func(instance)
    File "/Library/Python/2.7/site-packages/rest_framework/serializers.py", line 353, in _readable_fields
      field for field in self.fields.values()
  AttributeError: 'tuple' object has no attribute 'values'
  </code></pre>
  
  <p>my_app/urls.py:</p>
  
  <pre><code>urlpatterns = [
      url(r'^site-info/$', site_info, name='site_info'),
  ]
  </code></pre>
  
  <p>my_app/models.py:</p>
  
  <pre><code>from __future__ import unicode_literals
  
  from django.db import models
  
  # Create your models here.
  class SiteInfo(models.Model):
      site_id = models.IntegerField(primary_key=True)
      site_name = models.CharField(max_length=100, blank=True, null=True)
      latitude = models.DecimalField(max_digits=18, decimal_places=15, blank=True, null=True)
      longitude = models.DecimalField(max_digits=18, decimal_places=15, blank=True, null=True)
  
  class Meta:
      # managed = False
      db_table = 'site_info'
  </code></pre>
  
  <p>data/views:</p>
  
  <pre><code>from rest_framework.decorators import api_view
  from rest_framework.response import Response
  
  from .models import SiteInfo
  from .serializers import SiteInfoSerializer
  
  
  # Create your views here.
  @api_view(['GET', 'POST'])
  def site_info(request):
  """
      List all sites
  """
      if request.method == 'GET':
         sites = SiteInfo.objects.using('teldata').all()
         serializer = SiteInfoSerializer(sites, many=True)
         Response(serializer.data)
  </code></pre>
  
  <p>my_app/serializers:</p>
  
  <pre><code>from rest_framework import serializers
  
  from .models import SiteInfo
  
  
  class SiteInfoSerializer(serializers.ModelSerializer):
      model = SiteInfo
      fields = ('site_id', 'site_name', 'latitude', 'longitude')
  </code></pre>
  
  <p>what could be the problem? thanks</p>