title: Error with ordering Django 1.9 models how to fix this?
body: <p><strong>Output from CMD:</strong> </p>
  
  <blockquote>
    <p>django.core.management.base.SystemCheckError: SystemCheckError: System check identified some issues:</p>
    
    <p>ERRORS: posts.Comments: (models.E015) 'ordering' refers to the
    non-existent field 'timestamp'. posts.Comments: (models.E015)
    'ordering' refers to the non-existent field 'updated'.</p>
    
    <p>System check identified 2 issues (0 silenced).</p>
  </blockquote>
  
  <p><strong>Current code:</strong>  </p>
  
  <pre><code>from __future__ import unicode_literals
  from django.conf import settings
  from django.core.urlresolvers import reverse
  from django.db import models
  from django.db.models.signals import pre_save
  from django.utils import timezone
  
  from django.utils.text import slugify
  
  
  class PostManager(models.Manager):
      def active(self, *args, **kwargs):
          return super(PostManager, self).filter(draf=False).filter(publish__lte=timezone.now())
  
  def upload_location(instance, filename):
      return "%s/%s" %(instance.id, filename)
  
  class Post(models.Model):
      user                    = models.ForeignKey(settings.AUTH_USER_MODEL, default=1)
      title                   = models.CharField(max_length=70)
      slug                    = models.SlugField(unique=True)
      image                   = models.ImageField(upload_to=upload_location,
                                                  null=True,
                                                  blank=True, 
                                                  width_field="width_field", 
                                                  height_field="height_field")
      width_field             = models.IntegerField(default=0)
      height_field            = models.IntegerField(default=0)
      content                 = models.TextField(max_length=220)
      draf                    = models.BooleanField(default=False)
      publish                 = models.DateField(auto_now=False, auto_now_add=False)
      updated                 = models.DateTimeField(auto_now=True, auto_now_add=False)
      timestamp               = models.DateTimeField(auto_now=False, auto_now_add=True)
  
  class Comments(models.Model):
      class Meta:
          db_table        = 'comments'
  
      comments_text       = models.CharField(max_length=120)
      comments_posts      = models.ForeignKey(Post)
  
      objects = PostManager()
  
      def __str__(self):
          return self.title
  
      def get_absolute_url(self):
          return reverse("posts:detail", kwargs={"slug": self.slug})
          #return "/posts/%s/" %(self.id)
  
      class Meta:
          ordering = ["-timestamp", "-updated"]
  
  def create_slug(instance, new_slug=None):
      slug = slugify(instance.title)
      if new_slug is not None:
          slug = new_slug
      qs = Post.objects.filter(slug=slug).order_by("-id")
      exists = qs.exists()
      if exists:
          new_slug = "%s-%s" %(slug, qs.first().id)
          return create_slug(instance, new_slug=new_slug)
      return slug
  
  def pre_save_post_receiver(sender, instance, *args, **kwargs):
      if not instance.slug:
          instance.slug = create_slug(instance)
  
  pre_save.connect(pre_save_post_receiver, sender=Post)
  </code></pre>