Counterexample for http://twitter.com/#!/tpitale/status/29501998307

gistfile1.rb

ruby-1.8.7-p299 > class Vehicle
ruby-1.8.7-p299 ?>  include Mongoid::Document
ruby-1.8.7-p299 ?>  field :horsepower
ruby-1.8.7-p299 ?>  field :top_speed_mph
ruby-1.8.7-p299 ?>  end
 => nil 

ruby-1.8.7-p299 > Vehicle.new(:horsepower => 300, :top_speed_mph => 180).save
 => true 

ruby-1.8.7-p299 > Vehicle.find(:all).count
 => 1 

ruby-1.8.7-p299 > Vehicle.where(:horsepower => 300).first
 => #<Vehicle _id: 4cd07637aca25f5746000001, top_speed_mph: "180", horsepower: "300"> 

ruby-1.8.7-p299 > Vehicle.where(:horsepower.gt => 299).first
 => #<Vehicle _id: 4cd07637aca25f5746000001, top_speed_mph: "180", horsepower: "300"> 

ruby-1.8.7-p299 > Vehicle.where(:horsepower.gt => 299, :top_speed_mph.ne => "xyz").first
 => #<Vehicle _id: 4cd07637aca25f5746000001, top_speed_mph: "180", horsepower: "300">