```ruby
class Tweet < ActiveRecord::Base
  default_scope order("created_at desc")

  mount_uploader :image, ImageUploader

  def self.published
    where(published: true).where("published_at is not null")
  end
end
```

Sort desc by Twitter's timestamp
user
```ruby
class Tweet < ActiveRecord::Base
  default_scope order("published_at desc")

  mount_uploader :image, ImageUploader

  def self.published
    where(published: true).where("published_at is not null")
  end
end
```
assistant