order_by did't work after filtering by foreign key

I have a model like this

class Category(models.Model):      title = models.CharField(max_length=100, db_index=True)      slug = models.SlugField(max_length=100, db_index=True)        def __str__(self):          return "%s"%self.title    class Video(models.Model):      title = models.CharField(max_length=100, unique=True)      slug = models.SlugField(max_length=100, unique=True)      idlink = models.CharField(max_length=50)      posted = models.DateField(db_index=True, auto_now_add=True)      category = models.ForeignKey(Category)        def __str__(self):          return "%s"%self.title

And my views.py is

def list_video_cat(request, slug):      video = Video.objects.filter(category__slug=slug).order_by('-posted')      return  render(request, 'category.html', {'videos':video})

I want to show my post new to old, but it seems to did not work at this situation. What should I do?

P/S I read this topic before, but it does not solve my problem http://stackoverflow.com/questions/6114718/order-by-doesnt-work-with-filter-in-django-view

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