title: Unable to save Embedded field data in django admin
body: <p>When I try to save an embedded field through django admin, it gives me an error
  AttributeError at /admin/myapp/post/add/
  'NoneType' object has no attribute 'attname'
  I'm using djongomapper as db connector to django and MongoDB.</p>
  <pre><code>from djongo import models
  from django import forms
  
  # Create your models here.
  
  class Book(models.Model):
      title=models.CharField(max_length=200)
      author=models.CharField(max_length=200)
      
      class Meta:
          abstract = True
  
  class BookForm(forms.ModelForm):
      class Meta:
          model=Book
          fields = ('title','author')
  
  class Post(models.Model):
      heading = models.CharField(max_length=200)
      content = models.EmbeddedField(
          model_container = Book,
          model_form_class = BookForm
      )
      objects = models.DjongoManager()
  
  #admin.py
  from django.contrib import admin
  from .models import *
  
  admin.site.register(Post)
  </code></pre>
  <p><a href="https://i.stack.imgur.com/xfZpL.png" rel="nofollow noreferrer">django administration</a>
  <a href="https://i.stack.imgur.com/JUd03.png" rel="nofollow noreferrer">error when tried to save</a></p>
  <p>traceback:</p>
  <pre><code>Environment:
  
  
  Request Method: POST
  Request URL: http://127.0.0.1:8000/admin/myapp/post/add/
  
  Django Version: 3.1.3
  Python Version: 3.8.10
  Installed Applications:
  ['django.contrib.admin',
   'django.contrib.auth',
   'django.contrib.contenttypes',
   'django.contrib.sessions',
   'django.contrib.messages',
   'django.contrib.staticfiles',
   'myapp']
  Installed Middleware:
  ['django.middleware.security.SecurityMiddleware',
   'django.contrib.sessions.middleware.SessionMiddleware',
   'django.middleware.common.CommonMiddleware',
   'django.middleware.csrf.CsrfViewMiddleware',
   'django.contrib.auth.middleware.AuthenticationMiddleware',
   'django.contrib.messages.middleware.MessageMiddleware',
   'django.middleware.clickjacking.XFrameOptionsMiddleware']
  
  
  
  Traceback (most recent call last):
    File &quot;C:\Users\Admin\test\lib\site-packages\django\core\handlers\exception.py&quot;, line 47, in inner
      response = get_response(request)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\core\handlers\base.py&quot;, line 179, in _get_response
      response = wrapped_callback(request, *callback_args, **callback_kwargs)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\contrib\admin\options.py&quot;, line 614, in wrapper
      return self.admin_site.admin_view(view)(*args, **kwargs)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\utils\decorators.py&quot;, line 130, in _wrapped_view
      response = view_func(request, *args, **kwargs)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\views\decorators\cache.py&quot;, line 44, in _wrapped_view_func
      response = view_func(request, *args, **kwargs)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\contrib\admin\sites.py&quot;, line 233, in inner
      return view(request, *args, **kwargs)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\contrib\admin\options.py&quot;, line 1653, in add_view
      return self.changeform_view(request, None, form_url, extra_context)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\utils\decorators.py&quot;, line 43, in _wrapper
      return bound_method(*args, **kwargs)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\utils\decorators.py&quot;, line 130, in _wrapped_view
      response = view_func(request, *args, **kwargs)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\contrib\admin\options.py&quot;, line 1534, in changeform_view
      return self._changeform_view(request, object_id, form_url, extra_context)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\contrib\admin\options.py&quot;, line 1573, in _changeform_view
      form_validated = form.is_valid()
    File &quot;C:\Users\Admin\test\lib\site-packages\django\forms\forms.py&quot;, line 177, in is_valid
      return self.is_bound and not self.errors
    File &quot;C:\Users\Admin\test\lib\site-packages\django\forms\forms.py&quot;, line 172, in errors
      self.full_clean()
    File &quot;C:\Users\Admin\test\lib\site-packages\django\forms\forms.py&quot;, line 376, in full_clean
      self._post_clean()
    File &quot;C:\Users\Admin\test\lib\site-packages\django\forms\models.py&quot;, line 405, in _post_clean
      self.instance.full_clean(exclude=exclude, validate_unique=False)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\db\models\base.py&quot;, line 1209, in full_clean
      self.clean_fields(exclude=exclude)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\db\models\base.py&quot;, line 1251, in clean_fields
      setattr(self, f.attname, f.clean(raw_value, self))
    File &quot;C:\Users\Admin\test\lib\site-packages\django\db\models\fields\__init__.py&quot;, line 650, in clean
      value = self.to_python(value)
    File &quot;C:\Users\Admin\test\lib\site-packages\djongo\models\fields.py&quot;, line 259, in to_python
      f'Value: {value} must be an instance of {self.base_type}')
    File &quot;C:\Users\Admin\test\lib\site-packages\django\db\models\base.py&quot;, line 522, in __str__
      return '%s object (%s)' % (self.__class__.__name__, self.pk)
    File &quot;C:\Users\Admin\test\lib\site-packages\django\db\models\base.py&quot;, line 573, in _get_pk_val
      return getattr(self, meta.pk.attname)
  
  Exception Type: AttributeError at /admin/myapp/post/add/
  Exception Value: 'NoneType' object has no attribute 'attname'
  </code></pre>
  <p>Initial problem: unable to save it using django admin.</p>