Migrating DATA - not just schema, Rails

Question

Sometimes, data migrations are required. As time passes, code changes and migrations using your domain model are no longer valid and migrations fail. What are the best practices for migrating data?

I tried make an example to clarify the problem:

Consider this. You have a migration

class ChangeFromPartnerAppliedToAppliedAt < ActiveRecord::Migration
  def up
    User.all.each do |user|
      user.applied_at = user.partner_application_at
      user.save
   end
 end

this runs perfectly fine, of course. Later, you need a schema change

class AddAcceptanceConfirmedAt < ActiveRecord::Migration
  def change
    add_column :users, :acceptance_confirmed_at, :datetime
  end
end

class User < ActiveRecord::Base
  before_save :do_something_with_acceptance_confirmed_at
end

For you, no problem. It runs perfectly. But if your coworker pulls both these today, not having run the first migration yet, he'll get this error on running the first migration:

rake aborted!
An error has occurred, this and all later migrations canceled:
undefined method `acceptance_confirmed_at=' for #<User:0x007f85902346d8>

That's not being a team player, he'll be fixing the bug you introduced. What should you have done?

Answer 1

This is a perfect example of the Using Models in Your Migrations

class ChangeFromPartnerAppliedToAppliedAt < ActiveRecord::Migration
  class User < ActiveRecord::Base
  end

  def up
    User.all.each do |user|
      user.applied_at = user.partner_application_at
      user.save
   end
 end

Edited after Mischa's comment

class ChangeFromPartnerAppliedToAppliedAt < ActiveRecord::Migration
  class User < ActiveRecord::Base
  end

  def up
    User.update_all('applied_at = partner_application_at')
  end
 end