Can not login in django admin when using mongoDb

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 ./manage.py createsuperuser . Now when i am trying to login in django Admin it throws a error 'MetaDict' object has no attribute 'pk'

my settings.py

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&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', '[email protected]'),
)

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/'

Models.py

from django.db import models
from mongoengine import *

class Blog(Document):
    owner = StringField()
    title = StringField()
    # extensions = ListField(EmbeddedDocument(BlogExtension))
    tags = ListField(StringField())
    approved = BooleanField()

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