20150202171513_switch_type_of_state_to_enum.rb

class SwitchTypeOfStateToEnum < ActiveRecord::Migration
  class Order < ActiveRecord::Base
    enum state: [ :pending, :running, :success, :failed ]
  end

  def change
    rename_column :orders, :state, :state_old
    add_column :orders, :state, :integer

    reversible do |dir|
      dir.up do
       say_with_time 'Writing values into the new enum state column' do
          Order.all.each do |t|
            t.state = t.state_old
            t.save!
          end.size
        end
      end
      dir.down do
       say_with_time 'Writing values from enum back into the old state column' do
          Order.all.each do |t|
            t.state_old = t.state.to_s
            t.save!
          end.size
        end
      end
    end

    remove_column :orders, :state_old, :string
  end
end

README.md

Sample Migration for Rails 4.1 Enums

Rails 4.1 added enums to Active Record adding some convenient access and manipulation helpers for the typical status field. If you want to start using these in your legacy app today, you will not only have to update your models accordingly and add enum declarations, but you might also want to update your database schema and the contents of your production databases. This migration was written for just that usecase and schould be usable for your models and database with just some slight changes.