title: Error : "Document BlogPost may not be subclassed" mongoengine
body: <p>I am using pymongo version 2.6.1 with  mongoengine 0.8.4. And I want to create one code using tutorial given in</p>
  
  <p>link <a href="https://mongoengine-odm.readthedocs.org/en/latest/tutorial.html" rel="nofollow">https://mongoengine-odm.readthedocs.org/en/latest/tutorial.html</a></p>
  
  <p>My code is as follows in sample.py file: </p>
  
  <pre><code>from mongoengine import *
  import datetime
  
  class BlogPost(Document):
      title = StringField(required=True, max_length=200)
      posted = DateTimeField(default=datetime.datetime.now)
      tags = ListField(StringField(max_length=50))
  
  class TextPost(BlogPost):
      content = StringField(required=True)
  
  class LinkPost(BlogPost):
      url = StringField(required=True)
  </code></pre>
  
  <p>On terminal wnen I run , I get follwing error:</p>
  
  <p>Python 2.7.3 (default, Apr 10 2013, 05:46:21) 
  [GCC 4.6.3] on linux2
  Type "help", "copyright", "credits" or "license" for more information.</p>
  
  <blockquote>
    <blockquote>
      <blockquote>
        <p>from sample import Post
        Traceback (most recent call last):
          File "", line 1, in 
          File "sample.py", line 9, in 
            class TextPost(BlogPost):
          File "/usr/local/lib/python2.7/dist-packages/mongoengine/base/metaclasses.py", line 332, in <strong>new</strong>
            new_class = super_new(cls, name, bases, attrs)
          File "/usr/local/lib/python2.7/dist-packages/mongoengine/base/metaclasses.py", line 120, in <strong>new</strong>
            base.<strong>name</strong>) 
        <b>ValueError: Document BlogPost may not be subclassed</b></p>
      </blockquote>
    </blockquote>
  </blockquote>
  
  <p>Please help me. I also tried uninstall and reinstall. but it not works. </p>