django mptt ValueError:无法将None用作查询值

我正在使用django mptt创建目录类别模型，当我试图创建类别模型的第一个实例时，遇到了一个错误

>>> c = Category(name='cat1', slug='cat1')
>>> c.save()

错误：

ValueError at /admin/catalog/category/add/
Cannot use None as a query value 

仅当我未在类别实例中指向“父”字段时，才会出现此错误

class TimeStampedModel(models.Model):
    """
    Abstract class for adding 'create' and 'modified' info
    """
    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(auto_now=True)

    class Meta:
        abstract = True


@python_2_unicode_compatible
class Category(MPTTModel, TimeStampedModel):
    ''' Category model with MPTT storing approach '''
    name = models.CharField(max_length=128)
    slug = models.SlugField(max_length=50, unique=True)
    description = models.TextField(blank=True)
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children')
    is_active = models.BooleanField(default=True)

    def __str__(self):
        return self.name

    def get_absolute_url(self):
        return reverse('category', kwargs={'slug': self.slug})

    class MPTTMeta:
        order_insertion_by = ['id']

我找不到错误在哪里。你有什么建议吗

环境：

Request Method: POST
Request URL: http://127.0.0.1:8000/admin/catalog/category/add/

Django Version: 1.7.1
Python Version: 2.7.6
Installed Applications:
('django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'mptt',
 'sorl.thumbnail',
 'daguerre',
 'home',
 'utils',
 'accounts',
 'catalog',
 'debug_toolbar')
Installed Middleware:
(u'debug_toolbar.middleware.DebugToolbarMiddleware',
 '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')


Traceback:
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in wrapper
  584.                 return self.admin_site.admin_view(view)(*args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  105.                     response = view_func(request, *args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
  52.         response = view_func(request, *args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/contrib/admin/sites.py" in inner
  204.             return view(request, *args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in add_view
  1454.         return self.changeform_view(request, None, form_url, extra_context)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapper
  29.             return bound_func(*args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  105.                     response = view_func(request, *args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/utils/decorators.py" in bound_func
  25.                 return func.__get__(self, type(self))(*args2, **kwargs2)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/transaction.py" in inner
  394.                 return func(*args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in changeform_view
  1405.                 self.save_model(request, new_object, form, not add)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in save_model
  1046.         obj.save()
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/mptt/models.py" in save
  824.                         right_sibling = opts.get_ordered_insertion_target(self, parent)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/mptt/models.py" in get_ordered_insertion_target
  193.             queryset = node.__class__._tree_manager.filter(filters).order_by(*order_by)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/models/manager.py" in manager_method
  92.                 return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/models/query.py" in filter
  691.         return self._filter_or_exclude(False, *args, **kwargs)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/models/query.py" in _filter_or_exclude
  709.             clone.query.add_q(Q(*args, **kwargs))
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/models/sql/query.py" in add_q
  1287.         clause, require_inner = self._add_q(where_part, self.used_aliases)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/models/sql/query.py" in _add_q
  1309.                     current_negated)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/models/sql/query.py" in _add_q
  1314.                     current_negated=current_negated, connector=connector)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/models/sql/query.py" in build_filter
  1142.         value, lookups = self.prepare_lookup_value(value, lookups, can_reuse)
File "/home/oleshko/design/.virtualenvs/begemotik/local/lib/python2.7/site-packages/django/db/models/sql/query.py" in prepare_lookup_value
  1040.                 raise ValueError("Cannot use None as a query value")

Exception Type: ValueError at /admin/catalog/category/add/
Exception Value: Cannot use None as a query value

麻烦就在这里：

class MPTTMeta:
        order_insertion_by = ['id']

---

我将“order\u insertion\u by”字段更改为另一个字段（在我的示例中为“created”），它开始工作

我在引用MP\u节点子类中现在使用auto\u的字段时，通过使用“node\u order\u by”属性找到了此错误的原因

Class Tree(MP_Node):
    name = models.CharField(max_length=45)
    last_modified =  models.DateTimeField(auto_now=True)
    node_order_by = ['last_modified']

通过或使用其他字段注释掉节点\u order\u，错误消失。我认为问题在于Treebeard正试图找出在哪里插入子记录，但它还没有新记录的日期