migration_transactional_loop.rb

# This is a mechanism we use often in our migrations. When we have a migration
# that hits every record of a large table, this looping mechanism takes the
# bite away from the migration. This allows us to be transactional in our
# migration while avoiding aggressive locking of the table. Ultimately
# allowing us to do migratory deployments even more frequently during high
# usage areas of the day.
#
# Further, some long migrations that we'd run over a weekend may exceed the
# max transaction time set on our MySQL server. Running smaller transactions
# avoids this sensible restriction.
#
# We place this bit of code in config/initializers. Transactional loop requires
# will_paginate to execute.

class ActiveRecord::Migration
  def self.transactional_loop(active_record_class, paginate_options={}, &block)
    page = 1
    loop do
      no_more_records = true
      active_record_class.transaction do
        active_record_class.paginate({:page => page, :per_page => 100}.merge(paginate_options)).each do |active_record_instance|
          no_more_records = false
          block.call(active_record_instance)
        end
        page += page + 1
      end
      break if no_more_records
    end    
  end
end

usage.rb

class SomeMigration < ActiveRecord::Migration
  def self.up
    transactional_loop(Company, :include => :invoicing_profile) do |company|
      # do something to or with company
    end
  end
end