How to use created_at value as default in Rails

问题

How can I set one of my database table to created_at as default value. I did this but didn't work, I have checked in the DB and the value of code is null.

class CreateEvents < ActiveRecord::Migration
  def change
   create_table :events do |t|
    t.string :title
    t.string :location
    t.text :description
    t.date :occurs_on
    t.string :code, :default => Event::created_at

    t.timestamps

  end
 end
end

来源：https://stackoverflow.com/questions/9087494/how-to-use-created-at-value-as-default-in-rails