Adding indexes CONCURRENTLY in Rails

migration.rb

class AddIndexToUsersSubscriptionToken < ActiveRecord::Migration
  def self.up
    change_column_null :users, :subscription_token, false

    if ENV['NO_DDL_TRANSACTION']
      execute <<-eosql
        CREATE UNIQUE INDEX CONCURRENTLY index_users_on_subscription_token
        ON users (subscription_token)
      eosql
    else
      add_index :users, :subscription_token, :unique => true
    end
  end

  def self.down
    remove_index :users, :subscription_token
    change_column_null :users, :subscription_token, true
  end
end

rails-2-migrator-initializer.rb

class ActiveRecord::Migrator
  def ddl_transaction_with_override(&block)
    if ENV['NO_DDL_TRANSACTION']
      block.call
    else
      ddl_transaction_without_override &block
    end
  end

  alias_method_chain :ddl_transaction, :override
end