uniq.rb

class NotWorking
  include Comparable

  def initialize(obj)
    @attr = obj
  end

  attr_accessor :attr

  def <=>(other)
    if other.is_a? NotWorking
      attr <=> other.attr
    end
  end
end

class Working < NotWorking
  alias eql? ==

  def hash
    attr.hash
  end
end

p [NotWorking.new("foo"), NotWorking.new("bar"),
   NotWorking.new("foo")].uniq.size # 3

p [Working.new("foo"), Working.new("bar"),
   Working.new("foo")].uniq.size # 2