title: Django mptt, category and subcategory system - Category has no field named 'parent'
body: <p>I would like created category and subcategory system using django mptt.
  My try:</p>
  
  <pre><code>from django.db import models
  import mptt
  
  
  class Category(models.Model):
      name = models.CharField(max_length=255)
      category = models.ForeignKey('self', null=True, blank=True, related_name='children')
  
  mptt.register(Category, order_insertion_by=['name'])
  
  class Post(models.Model):
      title = models.CharField(max_length=255)
      text = models.TextField()
      category = models.ManyToManyField(Category)
  </code></pre>
  
  <p>My admin.py</p>
  
  <pre><code>from django.contrib import admin
  from xxxx.xxx.models import *
  from mptt.admin import MPTTModelAdmin
  
  admin.site.register(Category, MPTTModelAdmin)
  admin.site.register(Post)
  </code></pre>
  
  <p><strong>My error:</strong></p>
  
  <p>Category has no field named 'parent'        </p>