Adds a second migration phase to a Rails project for data migrations. Just like migrations, these can be run once and only once in each environment with a single command during every deployment.


Add this line to your application's Gemfile:

gem 'data_tasks'

And then execute:

$ bundle

Or install it yourself as:

$ gem install empty_gem


To generate a data migration, use the rails generator:

rails generate data_task MigrateSomeData

This will create:


Run all pending data tasks with rake:

rake data:migrate

Common Problems

If you're running your Rails processes in threadsafe mode, you'll get errors about undefined constants when trying to access model classes. You'll need to disable while running rake tasks. In your environment file (ex. config/environments/production.rb), change:



config.threadsafe! unless $rails_rake_task

If you don't want to make that change, you can explicitly load the required model classes inside the migration class, or define stub model classes. For example, if you wanted to add a new SideHatch record in the migration, you could define:

class SideHatch < ActiveRecord::Base; end

class CreateSideHatch < ActiveRecord::Migration
  def up
    SideHatch.create!(size: 49)


  • Add a capistrano hook to run data migrations after schema migrations
  • Add a heroku-san deployment strategy


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request