---
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"
---

**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
    #
    # @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
#app/models/user.rb
class User
  include Mongoid::Document
  include Mongoid::CreatedAt
# ...
end
u = User.new(created_at: 1.hour.ago)
u.created_at
```

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