title: Can not login in django admin when using mongoDb
body: <p>I am using MongoEngine(0.9) and pymango(2.8) with django (1.8).
  I create a database named "Dummy". when I create super user by <code>./manage.py createsuperuser</code> . Now when i am trying to login in django Admin it throws a error <strong><em>'MetaDict' object has no attribute 'pk'</em></strong></p>
  
  <p>my <strong>settings.py</strong></p>
  
  <pre><code>import os
  import mongoengine
  from mongoengine import connect
  import sys
  
  
  BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
  
  mongoengine.connect('dummy', host='localhost')
  
  
  SECRET_KEY = '_d!@y@jkwpnml%96amis1!)rz5&amp;e-jpr)s2#1yc+k77yqoyq@u'
  
  
  DEBUG = True
  
  ALLOWED_HOSTS = []
  SESSION_ENGINE = 'mongoengine.django.sessions'
  SESSION_SERIALIZER = 'mongoengine.django.sessions.BSONSerializer'
  
  
  INSTALLED_APPS = (
      'django.contrib.admin',
      'django.contrib.auth',
      'django.contrib.contenttypes',
      'django.contrib.sessions',
      'django.contrib.messages',
      'django.contrib.staticfiles',
      'rest_framework',
      'mongoengine.django.mongo_auth',
  
      'app'
  )
  SITE_ID='56c17b151eea0d0f74c29839'
  
  MIDDLEWARE_CLASSES = (
      'django.contrib.sessions.middleware.SessionMiddleware',
      'django.middleware.common.CommonMiddleware',
      'django.middleware.csrf.CsrfViewMiddleware',
      'django.contrib.auth.middleware.AuthenticationMiddleware',
      # 'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
      'django.contrib.messages.middleware.MessageMiddleware',
      'django.middleware.clickjacking.XFrameOptionsMiddleware',
      # 'django.middleware.security.SecurityMiddleware',
  )
  
  ROOT_URLCONF = 'TestAppProject.urls'
  
  
  WSGI_APPLICATION = 'TestAppProject.wsgi.application'
  
  
  
  ADMINS = (
      ('cp', 'cp@gmail.com'),
  )
  
  MANAGERS = ADMINS
  
  MONGODB_DATABASES = {
      'default': {'name': 'django_mongoengine'}
  }
  
  DATABASES = {
      'default': {
          'ENGINE': 'django.db.backends.dummy',
          # 'default':'sqlite://db/sqlite3.db',,
          # 'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
      }
  }
  
  
  
  
  AUTH_USER_MODEL = 'mongo_auth.MongoUser'
  
  
  
  
  
  
  
  LANGUAGE_CODE = 'en-us'
  
  TIME_ZONE = 'UTC'
  
  USE_I18N = True
  
  USE_L10N = True
  
  USE_TZ = True
  
  
  
  STATIC_URL = '/static/'
  </code></pre>
  
  <p>Models.py</p>
  
  <pre><code>from django.db import models
  from mongoengine import *
  
  class Blog(Document):
      owner = StringField()
      title = StringField()
      # extensions = ListField(EmbeddedDocument(BlogExtension))
      tags = ListField(StringField())
      approved = BooleanField()
  </code></pre>
  
  <p>Now I am stuck.Please help me what the error is this i searched a lot but i am unable to solve this error.
  Thanks in advance</p>