[28] pry(main)> tracking.checkpoints.where(:processing_port => 'origin', :tag.ne => nil).desc(:checkpoint_time)
=> #<Mongoid::Criteria
   selector: {"processing_port"=>"origin", "tag"=>{"$ne"=>nil}},
   options:  {:sort=>{"checkpoint_time"=>-1}},
   class:    Checkpoint,
   embedded: true>

[29] pry(main)> tracking.checkpoints.where(:processing_port => 'origin', :tag.ne => nil).desc(:checkpoint_time).first
=> #<Checkpoint _id: 4fe301072cfb39b016000056, _type: nil, created_at: 2012-06-21 19:09:59 +0800, updated_at: 2012-06-21 19:09:59 +0800,  tag: "Delivered", checkpoint_time: 2012-06-16 00:00:01 +0800, processing_port: "origin", coordinate: nil>

[30] pry(main)> tracking.checkpoints.where(:processing_port => 'origin', :tag.ne => nil).asc(:checkpoint_time)
=> #<Mongoid::Criteria
   selector: {"processing_port"=>"origin", "tag"=>{"$ne"=>nil}},
   options:  {:sort=>{"checkpoint_time"=>1}},
   class:    Checkpoint,
   embedded: true>

[31] pry(main)> tracking.checkpoints.where(:processing_port => 'origin', :tag.ne => nil).asc(:checkpoint_time).last
ArgumentError: comparison of Checkpoint with Checkpoint failed
from /Users/Dante/.rvm/gems/ruby-1.9.3-p194@global/gems/mongoid-3.0.0.rc/lib/mongoid/contextual/memory.rb:358:in `sort!'