new blog post Mongoid: Use ObjectId as created_at

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

@@ -0,0 +1,44 @@
+---
+layout: post
+title: "Mongoid: Use ObjectId as created_at"
+date: 2013-08-23 07:34
+comments: true
+categories:
+- mongodb
+- rails
+- mongoid
+description: "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.
+
+```ruby
+#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.
+
+```ruby
+#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?