optimistic_locking.rb

module OptimisticLocking
  class StaleObjectError < Exception
  end

  def update
    dirty_attributes = self.dirty_attributes
    return true if dirty_attributes.empty?
    count = repository.update(dirty_attributes, to_query)
    if count == 1
      return true
    else
      raise StaleObjectError
    end
  end

  # hackery to get the timestamp into the update clause
  def to_query(query = {})
    qry = super
    unless self.new?
      lock_version = self.original_attributes[:timestamp]
      if lock_version
        qry.send(:append_condition, :timestamp, lock_version)
      end
    end
    qry
  end
end