blog/source/_posts/2013-08-23-mongoid-use-objectid-as-created-at.markdown

layout	title	date	comments	categories	description
post	Mongoid: Use ObjectId as created_at	2013-08-23 07:34	true	mongodb rails mongoid	Mongoid: Use ObjectId as created_at

One great feature of Mongodb is, that the first bytes of each ObjectID contains the time, they were generated. This can be exploited to mimic the well known created_at field in rails. First put this file in your lib directory.

#lib/mongoid/created.rb
module Mongoid
  module CreatedAt
    # Returns the creation time calculated from  ObjectID
    #
    # @return [ Date ] the creation time
    def created_at
      id.generation_time
    end
  end
end

Now you can include this module in every Model, where you need created at.

#app/models/user.rb
class User
  include Mongoid::Document
  include Mongoid::CreatedAt
# ...
end
u = User.new
u.created_at

That's all easy enough, isn't it?