mongoid/created_at: add setter

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

@@ -10,24 +10,38 @@ categories:
 description: "Mongoid: Use ObjectId as created_at"
 ---
 
+**Update: Nov 20 2014:** add setter
+
 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
+    # Returns the creation time calculated from ObjectID
     #
     # @return [ Date ] the creation time
     def created_at
       id.generation_time
     end
+
+    # Set generation time of ObjectId.
+    # Note: This will modify the ObjectId and
+    # is therefor only useful for not persisted documents
+    #
+    # @return [ BSON::ObjectId ] the generated object id
+    def created_at=(date)
+      self.id = BSON::ObjectId.from_time(date)
+    end
   end
 end
 ```
 
+If you are still using mongoid 3 replace `BSON::ObjectId` with `Moped::BSON::ObjectId`.
+
 Now you can include this module in every Model, where you need created at.
 
 ```ruby
@@ -37,7 +51,7 @@ class User
   include Mongoid::CreatedAt
 # ...
 end
-u = User.new
+u = User.new(created_at: 1.hour.ago)
 u.created_at
 ```