order.rb

class Order
  include Mongoid::Document

  field :state
  field :sent_at, type: Time
  field :number, type: Integer

  index({ sent_at: 1 })

  scope :sent, any_in(state: [:sent]).desc(:sent_at)

end

output.rb

Loading development environment (Rails 3.2.9)
1.9.3p327 :001 > Order.delete_all
  MOPED: 127.0.0.1:27017 COMMAND      database=admin command={:ismaster=>1} (1.5669ms)
  MOPED: 127.0.0.1:27017 COMMAND      database=echo_dev command={:count=>"orders", :query=>{}} (0.4501ms)
  MOPED: 127.0.0.1:27017 DELETE       database=echo_dev collection=orders selector={} flags=[] (0.1540ms)
 => 3 
1.9.3p327 :002 > Order.create!(state: :sent, sent_at: Time.now, number: 100)
  MOPED: 127.0.0.1:27017 INSERT       database=echo_dev collection=orders documents=[{"_id"=>"5130ffd27c71c15822000001", "state"=>:sent, "sent_at"=>2013-03-01 19:21:54 UTC, "number"=>100}] flags=[] (0.2131ms)
 => #<Order _id: 5130ffd27c71c15822000001, state: :sent, sent_at: 2013-03-01 19:21:54 UTC, number: 100> 
1.9.3p327 :003 > Order.create!(state: :sent, sent_at: Time.now, number: 1)
  MOPED: 127.0.0.1:27017 INSERT       database=echo_dev collection=orders documents=[{"_id"=>"5130ffde7c71c15822000002", "state"=>:sent, "sent_at"=>2013-03-01 19:22:06 UTC, "number"=>1}] flags=[] (0.2470ms)
 => #<Order _id: 5130ffde7c71c15822000002, state: :sent, sent_at: 2013-03-01 19:22:06 UTC, number: 1> 
1.9.3p327 :004 > Order.create!(state: :not_sent, sent_at: Time.now, number: 120)
  MOPED: 127.0.0.1:27017 INSERT       database=echo_dev collection=orders documents=[{"_id"=>"5130ffe47c71c15822000003", "state"=>:not_sent, "sent_at"=>2013-03-01 19:22:12 UTC, "number"=>120}] flags=[] (0.2091ms)
 => #<Order _id: 5130ffe47c71c15822000003, state: :not_sent, sent_at: 2013-03-01 19:22:12 UTC, number: 120> 
1.9.3p327 :005 > Order.sent.max(:number)
  MOPED: 127.0.0.1:27017 COMMAND      database=echo_dev command={:count=>"orders", :query=>{"state"=>{"$in"=>[:sent]}}} (1.0071ms)
  MOPED: 127.0.0.1:27017 COMMAND      database=echo_dev command={:aggregate=>"orders", :pipeline=>[{"$match"=>{"state"=>{"$in"=>[:sent]}, "number"=>{"$nin"=>[nil]}}}, {"$group"=>{"_id"=>"number", "count"=>{"$sum"=>1}, "max"=>{"$max"=>"$number"}, "min"=>{"$min"=>"$number"}, "sum"=>{"$sum"=>"$number"}, "avg"=>{"$avg"=>"$number"}}}]} (0.7901ms)
 => 100 
1.9.3p327 :006 >