order_criteria = Order.desc(:name)
 => #<Mongoid::Criteria
  selector: {:deleted_at=>nil},
  options:  {:sort=>[[:name, :desc]]},
  class:    Order,
  embedded: false>
 
order_criteria.first
 => #<Order _id: 4e8e8b169759f77a73000001 ... > 

order_criteria
 => #<Mongoid::Criteria
  selector: {:deleted_at=>nil},
  options:  {:sort=>[[:name, :desc], [:_id, :asc]]},
  class:    Order,
  embedded: false>

order_criteria = Order.desc(:_id)
 => #<Mongoid::Criteria
  selector: {:deleted_at=>nil},
  options:  {:sort=>[[:_id, :desc]]},
  class:    Order,
  embedded: false>
 
order_criteria.first
 => #<Order _id: 4e8e8b169759f77a73000001 ... > 

order_criteria
 => #<Mongoid::Criteria
  selector: {:deleted_at=>nil},
  options:  {:sort=>[[:_id, :desc], [:_id, :asc]]},
  class:    Order,
  embedded: false>