title: Local field 'created_at' in class 'Attachment' clashes with field of similar name from base class 'Timestampable'
body: <p>I have the following two models:</p>
  
  <pre><code>class Timestampable(models.Model):
      created_at = models.DateTimeField(null=True, default=None)
      updated_at = models.DateTimeField(null=True, default=None)
  
      class Meta:
          abstract = True
  
      def save(self, *args, **kwargs):
          now = timezone.now()
          if not self.created_at:
              self.created_at = now
          self.updated_at = now
  
          super(Timestampable, self).save(*args, **kwargs)
  
  class Attachment(Timestampable, models.Model):
      uuid = models.CharField(max_length=64, unique=True)
      customer = models.CharField(max_length=64)
      user = models.CharField(max_length=64)
      file = models.FileField(upload_to=upload_to)
      filename = models.CharField(max_length=255)
      mime = models.CharField(max_length=255)
      publicly_accessible = models.BooleanField(default=False)
  </code></pre>
  
  <p>When I try to migrate these models, I get the following error:</p>
  
  <pre><code>django.core.exceptions.FieldError: Local field 'created_at' in class 'Attachment' clashes with field of similar name from base class 'Timestampable'
  </code></pre>
  
  <p>I read <a href="https://stackoverflow.com/questions/15290750/fielderror-local-field-password-in-class-user-clashes-with-field-of-similar">here</a>, <a href="https://stackoverflow.com/questions/28662686/local-field-clashes-with-field-of-similar-name">here</a>, and <a href="https://stackoverflow.com/questions/27468136/django-custom-user-field-clashes-with-abstractbaseuser">here</a> that this should work when the base class is abstract. However, I marked it as abstract it still it doesn't seem to work. What else could be wrong? I am using Django 1.8.14.</p>