rails--rails/activerecord/lib/active_record/locking.rb

module ActiveRecord
  # Active Records support optimistic locking if the field <tt>lock_version</tt> is present.  Each update to the
  # record increments the lock_version column and the locking facilities ensure that records instantiated twice
  # will let the last one saved raise a StaleObjectError if the first was also updated. Example:
  #
  #   p1 = Person.find(1)
  #   p2 = Person.find(1)
  #   
  #   p1.first_name = "Michael"
  #   p1.save
  #   
  #   p2.first_name = "should fail"
  #   p2.save # Raises a ActiveRecord::StaleObjectError
  #
  # You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging,
  # or otherwise apply the business logic needed to resolve the conflict.
  #
  # You must ensure that your database schema defaults the lock_version column to 0.
  #
  # This behavior can be turned off by setting <tt>ActiveRecord::Base.lock_optimistically = false</tt>.
  module Locking
    def self.append_features(base) #:nodoc:
      super
      base.class_eval do
        alias_method :update_without_lock, :update
        alias_method :update, :update_with_lock
      end
    end

    def update_with_lock #:nodoc:
      if locking_enabled?
        previous_value    = self.lock_version
        self.lock_version = previous_value + 1
        
        affected_rows = connection.update(<<-end_sql, "#{self.class.name} Update with optimistic locking")
          UPDATE #{self.class.table_name}
          SET #{quoted_comma_pair_list(connection, attributes_with_quotes(false))}
          WHERE #{self.class.primary_key} = #{quote(id)} AND lock_version = #{quote(previous_value)}
        end_sql

        unless affected_rows == 1
          raise ActiveRecord::StaleObjectError, "Attempted to update a stale object"
        end
      else
        update_without_lock
      end
    end
  end

  class Base
    @@lock_optimistically = true
    cattr_accessor :lock_optimistically

    def locking_enabled? #:nodoc:
      lock_optimistically && respond_to?(:lock_version)
    end
  end
end