Django throws an exception when I add data from admin interface

[Yu Yang]

It is a blog app, I have created a Article model and Tag model. when I add data to Article table from Admin interface, the data is stored to database, but if I click the link to change the entry, it throws an exception "string indices must be integers, not str", the traceback is as follows:
Environment:


Request Method: GET
Request URL: http://localhost:8000/admin/blog/article/1/

Django Version: 1.5.1
Python Version: 2.7.3
Installed Applications:
('django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.admin',
 'blog')
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware')


Traceback:
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  115.                         response = callback(request, *callback_args, **callback_kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in wrapper
  372.                 return self.admin_site.admin_view(view)(*args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
  89.         response = view_func(request, *args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/contrib/admin/sites.py" in inner
  202.             return view(request, *args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapper
  25.             return bound_func(*args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/utils/decorators.py" in bound_func
  21.                 return func(self, *args2, **kwargs2)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/db/transaction.py" in inner
  223.                 return func(*args, **kwargs)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in change_view
  1112.             form = ModelForm(instance=obj)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/forms/models.py" in __init__
  244.             object_data = model_to_dict(instance, opts.fields, opts.exclude)
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/forms/models.py" in model_to_dict
  129.                 data[f.name] = list(f.value_from_object(instance).values_list('pk', flat=True))
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/db/models/fields/related.py" in value_from_object
  1333.         return getattr(obj, self.attname).all()
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/db/models/manager.py" in all
  128.         return self.get_query_set()
File "/home/yangyu/.virtualenvs/django-blog/local/lib/python2.7/site-packages/django/db/models/fields/related.py" in get_query_set
  612.                 return self.instance._prefetched_objects_cache[self.prefetch_cache_name]

Exception Type: TypeError at /admin/blog/article/1/
Exception Value: string indices must be integers, not str


the code of Article model and Tag model is as follows:

class Tag(models.Model):
    name = models.CharField(max_length=50, unique=True, verbose_name='标签')
    slug = models.SlugField()

    def __unicode__(self):
        return self.name

    class Meta:
        verbose_name = '标签'
        verbose_name_plural = '标签'

    @models.permalink
    def get_absolute_url(self):
        return ('blog_tag', (), {'idx': self.id, 'slug': self.slug})


# Create your models here.
class Article(models.Model):
    """
    the model for blog article
    """
    STATUS_CHOICE = (
        (1, '编辑'),
        (2, '完成'),
        (3, '失效'),
    )
    title = models.CharField(max_length=100, verbose_name='标题')
    slug = models.SlugField(max_length=100)
    content = models.TextField(verbose_name='内容')
    created_time = models.DateTimeField(auto_now_add=True, verbose_name="创建时间")
    modified_time = models.DateTimeField(default=datetime.now,
                                         verbose_name="修改时间")
    tags = models.ManyToManyField(Tag, verbose_name='标签')
    category = models.ForeignKey('Category', blank=True, null=True, verbose_name='分类')
    always_top = models.BooleanField(default=False, verbose_name='置顶')
    author = models.CharField(max_length=40, verbose_name='作者')
    status = models.IntegerField(choices=STATUS_CHOICE,
                                 default=1, verbose_name='状态')

    @models.permalink
    def get_absolute_url(self):
        return ('blog_article', (), {'idx': self.id, 'slug': self.slug})

    def __getattr__(self, name):
        """
        add two attributes(summary, visible_comments) to the object
        Arguments:
        - `name`:
        """
        if name == 'summary':
            return util.get_summary(self.content)
        else:
            return ''

    def __unicode__(self):
        return self.title

    class Meta:
        ordering = ['-always_top', '-created_time']
        verbose_name = '博客文章'
        verbose_name_plural = '博客文章'

the problem seems to be caused by "tags = models.ManyToManyField(Tag, verbose_name='标签')" , when I delete this line, there is no problem, 
but when I add it to the model, django throws an expcetion, this problem really annoys me, who can help me?

[Tom Evans]

This __getattr__ method is utterly bogus.

With this method, every attempt to get an attribute of this instance
either returns something special if the attribute is named 'summary',
but most importantly, **it returns an empty string for EVERYTHING
else**, regardless of anything, whether the attribute exists or not.

I imagine at some point something in django is looking at an attribute
which should be None, or raise an AttributeError, but it doesn't, it
returns an empty string. It then uses this bogus empty string to try
and index into a string.

I imagine removing the ManyToManyField bypasses the code that does this.

Cheers

Tom

[Yu Yang]

Thanks for your reply, you are right.