module CompositePrimaryKeys
  ID_SEP = ','
  module ArrayExtension
    def to_composite_keys
      CompositeKeys.new(self)
    end
    
    def to_composite_ids
      CompositeIds.new(self)
    end
  end
  
  class CompositeArray < Array
    def to_s
      join(ID_SEP)
    end
  end  

  class CompositeKeys < CompositeArray

  end

  class CompositeIds < CompositeArray

  end
end

Array.send(:include, CompositePrimaryKeys::ArrayExtension)