ruby script/plugin install http://projects.netlab.jp/svn/rails_plugins/pessimistic_locking

You can specify the new option :lock of ActiveRecord::Base.find() to lock rows:

    Account.transaction do
      shugo = Account.find(:first, :conditions => "name = 'shugo'", :lock => true)
      yuko = Account.find(:first, :conditions => "name = 'yuko'", :lock => true)
      shugo.balance -= 100
      shugo.save
      yuko.balance += 100
      yuko.save
    end

Or you can also use ActiveRecord::Base#lock() instead:

    Account.transaction do
      accounts = Account.find(:all, :conditions => ...)
      account1 = accounts.detect { |account| ... }
      account2 = accounts.detect { |account| ... }
      account1.lock
      account2.lock
      account1.balance -= 100
      account1.save
      account2.balance += 100
      account2.save
    end

The latter way may be better if you don't need lock all SELECTed rows.

There is currently no advanced documentation for this plugin.