title: Document Serializer returns object instead of id while using listfield of referencefield
body: <p>I am using django 1.5.11 for mongodb. Serializer.data is returning objects instead of id for all references given in listfield. 
  I am posting data in json format like this:</p>
  
  <pre><code>{
      "author_id": ["5874a85b58c1d23a5343cc87","5874a85058c1d23a5343cc86"],
      "title":"my book1"
  }
  </code></pre>
  
  <p>print serializer.data gives:</p>
  
  <pre><code>{'author_id': [&lt;Authors: Authors object&gt;, &lt;Authors: Authors object&gt;], 'Cost': None, 'id': u'5874c75a58c1d23ec8adf40f', 'title': u'my book1'}
  </code></pre>
  
  <p>I am expecting serializer.data as:</p>
  
  <pre><code>{'id': u'5874c75a58c1d23ec8adf40f', u'author_id': [u'5874a85b58c1d23a5343cc87', u'5874a85058c1d23a5343cc86'], u'title': u'my book1'}
  </code></pre>
  
  <p>My files are as follows:</p>
  
  <p>model.py:</p>
  
  <pre><code>from django.db import models
  from rest_framework_mongoengine.serializers import DocumentSerializer
  from mongoengine.document import Document
  from mongoengine.fields import EmbeddedDocumentField,ListField,ReferenceField, StringField, ObjectIdField, IntField, BooleanField, FloatField, DateTimeField
  from mongoengine import connect, CASCADE
  from pymongo import ReadPreference
  
  
  connect('mydb16', username='admin', password='admin123')
  
  class Authors(Document):    
      author_name = StringField(max_length=30)
      author_country = StringField(max_length=30)
  
  class Books(Document):
      author_id =ListField(ReferenceField('Authors',required = True))
      #author_id =ReferenceField('Authors')
      title = StringField(max_length=30)
      Cost = IntField()
  </code></pre>
  
  <p>serializer.py:</p>
  
  <pre><code>from app.models import Authors
  from app.models import Books
  
  from rest_framework_mongoengine.serializers import DocumentSerializer
  from rest_framework import serializers
  
  class AuthorsSerializer(DocumentSerializer):
  
      class Meta:
          model = Authors
      def to_internal_value(self, data):
          return super(DocumentSerializer, self).to_internal_value(data)
  
  
  
  class BooksSerializer(DocumentSerializer):
      class Meta:
          model = Books
  
      def to_internal_value(self, data):
          return super(DocumentSerializer, self).to_internal_value(data)      
  </code></pre>
  
  <p>views.py</p>
  
  <pre><code>from app.serializers import AuthorsSerializer
  from app.serializers import BooksSerializer
  from rest_framework import status
  from app.models import Authors
  from app.models import Books
  from rest_framework.views import APIView
  from rest_framework.response import Response
  
  class AuthorsList(APIView):
      def get(self,request,fromat=None):
          print "I am Authorlist get..."    
          author = Authors.objects.all()               
          serializer = AuthorsSerializer(author, many=True)
          print "\n\nserializers data: " , serializer.data, "\n\n"
          return Response(serializer.data)
  
      def post(self, request, format=None):
          print "I am Authorlist post"    
          data = request.data
          print "\nData: ", data , "\n\n"        
          serializer = AuthorsSerializer(data=data)
  
  
          if serializer.is_valid():   
              serializer.save()
  
              return Response(serializer.data, status=status.HTTP_201_CREATED)
          return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
  
  class BooksList(APIView):
  
      def get(self,request,format=None):
          print "I am Bookslist get"    
          books = Books.objects.all()      
          print "books :",books         
          serializer = BooksSerializer(books, many=True)
          print "serializer :",serializer
          print "serializer.data :",serializer.data
          return Response(serializer.data)        
  
      def post(self, request, format=None):
          print "I am Bookslist post"    
          data=request.data
          print "data :" , data
          print "\nData: ", data , "\n\n"
  
          serializer = BooksSerializer(data=data)
  
          if serializer.is_valid():
              serializer.save()
              print "serializer.data ", serializer.data
              return Response(serializer.data, status=status.HTTP_201_CREATED)
          return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST) 
  </code></pre>
  
  <p>urls.py:</p>
  
  <pre><code>from django.conf.urls import patterns, url
  import views
  from django.contrib import admin
  
  
  
  urlpatterns = [
      url(r'^admin/', admin.site.urls),
      url(r'^authors/$', views.AuthorsList.as_view()),
     # url(r'^books/(?P&lt;pk&gt;[0-9a-f]+)/$', views.BooksList.as_view()), 
      url(r'^books/$', views.BooksList.as_view()),   
  
  ]
  </code></pre>
  
  <p>settings.py:</p>
  
  <pre><code># Django settings for new_project project.
  
  DEBUG = True
  TEMPLATE_DEBUG = DEBUG
  
  ADMINS = (
      # ('Your Name', 'your_email@example.com'),
  )
  
  MANAGERS = ADMINS
  
  DATABASES = {
      'default': {
          'ENGINE': 'django_mongodb_engine', # Add 'postgresql_psycopg2', 'mysql', 'sqlite3' or 'oracle'.
          'NAME': 'mydb16',                      # Or path to database file if using sqlite3.
          # The following settings are not used with sqlite3:
          'USER': 'admin',
          'PASSWORD': 'admin123',
          'HOST': '127.0.0.1',                      # Empty for localhost through domain sockets or '127.0.0.1' for localhost through TCP.
          'PORT': '27017',                      # Set to empty string for default.
      }
  }
  
  # Hosts/domain names that are valid for this site; required if DEBUG is False
  # See https://docs.djangoproject.com/en/1.5/ref/settings/#allowed-hosts
  ALLOWED_HOSTS = []
  
  # Local time zone for this installation. Choices can be found here:
  # http://en.wikipedia.org/wiki/List_of_tz_zones_by_name
  # although not all choices may be available on all operating systems.
  # In a Windows environment this must be set to your system time zone.
  TIME_ZONE = 'America/Chicago'
  
  # Language code for this installation. All choices can be found here:
  # http://www.i18nguy.com/unicode/language-identifiers.html
  LANGUAGE_CODE = 'en-us'
  
  SITE_ID = u'5874a7aa58c1d23a3040bb5f'
  
  # If you set this to False, Django will make some optimizations so as not
  # to load the internationalization machinery.
  USE_I18N = True
  
  # If you set this to False, Django will not format dates, numbers and
  # calendars according to the current locale.
  USE_L10N = True
  
  # If you set this to False, Django will not use timezone-aware datetimes.
  USE_TZ = True
  
  # Absolute filesystem path to the directory that will hold user-uploaded files.
  # Example: "/var/www/example.com/media/"
  MEDIA_ROOT = ''
  
  # URL that handles the media served from MEDIA_ROOT. Make sure to use a
  # trailing slash.
  # Examples: "http://example.com/media/", "http://media.example.com/"
  MEDIA_URL = ''
  
  # Absolute path to the directory static files should be collected to.
  # Don't put anything in this directory yourself; store your static files
  # in apps' "static/" subdirectories and in STATICFILES_DIRS.
  # Example: "/var/www/example.com/static/"
  STATIC_ROOT = ''
  
  # URL prefix for static files.
  # Example: "http://example.com/static/", "http://static.example.com/"
  STATIC_URL = '/static/'
  
  # Additional locations of static files
  STATICFILES_DIRS = (
      # Put strings here, like "/home/html/static" or "C:/www/django/static".
      # Always use forward slashes, even on Windows.
      # Don't forget to use absolute paths, not relative paths.
  )
  
  # List of finder classes that know how to find static files in
  # various locations.
  STATICFILES_FINDERS = (
      'django.contrib.staticfiles.finders.FileSystemFinder',
      'django.contrib.staticfiles.finders.AppDirectoriesFinder',
  #    'django.contrib.staticfiles.finders.DefaultStorageFinder',
  )
  
  # Make this unique, and don't share it with anybody.
  SECRET_KEY = '7x17q1(2v(r&amp;6)66)cjwfql#h@ng#+do90fi-(yj74k)e=gli0'
  
  # List of callables that know how to import templates from various sources.
  TEMPLATE_LOADERS = (
      'django.template.loaders.filesystem.Loader',
      'django.template.loaders.app_directories.Loader',
  #     'django.template.loaders.eggs.Loader',
  )
  
  MIDDLEWARE_CLASSES = (
      'django.middleware.common.CommonMiddleware',
      'django.contrib.sessions.middleware.SessionMiddleware',
      'django.middleware.csrf.CsrfViewMiddleware',
      'django.contrib.auth.middleware.AuthenticationMiddleware',
      'django.contrib.messages.middleware.MessageMiddleware',
      # Uncomment the next line for simple clickjacking protection:
      # 'django.middleware.clickjacking.XFrameOptionsMiddleware',
  )
  
  ROOT_URLCONF = 'new_project.urls'
  
  # Python dotted path to the WSGI application used by Django's runserver.
  WSGI_APPLICATION = 'new_project.wsgi.application'
  
  TEMPLATE_DIRS = (
      # Put strings here, like "/home/html/django_templates" or "C:/www/django/templates".
      # Always use forward slashes, even on Windows.
      # Don't forget to use absolute paths, not relative paths.
  )
  
  INSTALLED_APPS = (
      'django.contrib.auth',
      'django.contrib.contenttypes',
      'django.contrib.sessions',
      'django.contrib.sites',
      'django.contrib.messages',
      'django.contrib.staticfiles',
      'rest_framework',
      'app',
      'django.contrib.admin',
      # Uncomment the next line to enable the admin:
      # 'django.contrib.admin',
      # Uncomment the next line to enable admin documentation:
      # 'django.contrib.admindocs',
  )
  
  SESSION_SERIALIZER = 'django.contrib.sessions.serializers.JSONSerializer'
  
  # A sample logging configuration. The only tangible logging
  # performed by this configuration is to send an email to
  # the site admins on every HTTP 500 error when DEBUG=False.
  # See http://docs.djangoproject.com/en/dev/topics/logging for
  # more details on how to customize your logging configuration.
  LOGGING = {
      'version': 1,
      'disable_existing_loggers': False,
      'filters': {
          'require_debug_false': {
              '()': 'django.utils.log.RequireDebugFalse'
          }
      },
      'handlers': {
          'mail_admins': {
              'level': 'ERROR',
              'filters': ['require_debug_false'],
              'class': 'django.utils.log.AdminEmailHandler'
          }
      },
      'loggers': {
          'django.request': {
              'handlers': ['mail_admins'],
              'level': 'ERROR',
              'propagate': True,
          },
      }
  }
  </code></pre>
  
  <p>I have installed django in virtual environment. My pip freeze is as below:</p>
  
  <pre><code>Django==1.5.11
  django-browserid==2.0.2
  django-classy-tags==0.8.0
  django-missing==0.1.18
  django-mongo-auth==0.1.3
  django-mongodb-engine==0.6.0
  django-redis-sessions==0.5.6
  django-rest-framework-mongoengine==3.3.0
  django-sekizai==0.10.0
  django-websocket-redis==0.4.7
  djangorestframework==3.1.2
  djangorestframework-jwt==1.9.0
  djangotoolbox==1.8.0
  gevent==1.1.2
  greenlet==0.4.10
  mongoengine==0.9.0
  oauthlib==2.0.1
  pika==0.10.0
  Pygments==2.1.3
  PyJWT==1.4.2
  pymongo==2.8
  python-dateutil==2.6.0
  python-openid==2.2.5
  pytz==2016.10
  redis==2.10.5
  requests==2.12.3
  requests-oauthlib==0.7.0
  rest-condition==1.0.3
  six==1.10.0
  tweepy==3.5.0
  </code></pre>
  
  <p>I have been stuck here, any help will be greatly appreciated.
  Thanks in advance...</p>