create_semaphores.rb

class CreateSemaphores < ActiveRecord::Migration
   def self.up
      create_table :semaphores do |t|
         t.column :name, :text
         t.column :value, :integer
      end
      add_index :semaphores, :name, :unique => true
   end
end

sample_usage.rb

Semaphore.lock('lock1', 'lock2') do
   # code
end

semaphore.rb

class Semaphore < ActiveRecord::Base
   def self.lock(*keys)
      transaction do
         keys.sort.map { |key| create!(:name => key).destroy }
         yield
      end
   end
end