class Job
  include Mongoid::Document
  include Mongoid::Timestamps

  default_scope desc(:created_at)
end

Job.first 
# MOPED: 127.0.0.1:27017 QUERY ... 
#   selector={"$query"=>{}, "$orderby"=>{"created_at"=>-1}} 
#   flags=[:slave_ok] limit=-1 skip=0 fields=nil (0.0ms) 
Job.last 
# MOPED: 127.0.0.1:27017 QUERY ... 
#   selector={"$query"=>{}, "$orderby"=>{:_id=>-1}} 
#   flags=[:slave_ok] limit=-1 skip=0 fields=nil (0.0ms)