Online (non-blocking) ActiveRecord migrations using pt-online-schema-change utility from Percona Toolkit

online_change_table.rb

# Online (non-blocking) ActiveRecord migrations using pt-online-schema-change utility from Percona Toolkit.
#
# In your Rails app put it as lib/online_change_table.rb
# Now you can use it in your migration file:
#
#    require 'online_change_table'
#
#    class AddSomethingToSomethings < ActiveRecord::Migration
#      def up
#        online_change_table :somethings do |t|
#          t.column :a, :string
#          t.column :b, :integer, default: 0
#          t.change :c, :text, default: '...'
#        end
#      end
#    end
#
# Tested only on ActiveRecord 4.1
# Maybe some time i will release it as a gem :D
 
class ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter
  def online_change_table(table_name)
    recorder = ActiveRecord::Migration::CommandRecorder.new(self)
    yield update_table_definition(table_name, recorder)
 
    operations = recorder.commands
 
    sqls = operations.map do |command, args|
      table, arguments = args.shift, args
      method = :"#{command}_sql"
 
      if respond_to?(method, true)
        send(method, table, *arguments)
      else
        raise "Unknown method called : #{method}(#{arguments.inspect})"
      end
    end.flatten.join(", ")
 
    config = ActiveRecord::Base.configurations[Rails.env]
 
    dsn = "D=#{config['database']},t=#{table_name}"
    dsn << ",h=#{config['host']}" if config['host']
    dsn << ",u=#{config['username']}" if config['username']
    dsn << ",p=#{config['password']}" if config['password']
    dsn << ",P=#{config['port']}" if config['port']
    dsn << ",A=#{config['encoding']}" if config['encoding']
 
    ok = system 'pt-online-schema-change', '--alter', sqls, dsn, '--execute'
    fail unless ok
  end
end