require 'active_record'

class Document < ActiveRecord::Base
  named_scope :by_state, lambda { |state| {:conditions => ['orders.state = ?',state]} }

   STATES = %w{queued working finished failed}.freeze

   STATES.each do |state|
     define_method("#{state}?") { self.state == state }
   end

end

documents = Document.by_state('working').find(:all)
documents.each do |doc|
  doc.state #=> 'working'
  doc.working? #=> true
  doc.finished? #=> false
end