http://javieracero.com/blog/the-key-to-ruby-hashes-is-eql-hash

hash2.rb

class Key
  include Comparable

  attr_reader :value

  def initialize(value)
    @value = value
  end 

  def <=>(other_key)
    value <=> other_key.value
  end 
end

k1 = Key.new('foo')
k2 = Key.new('foo')

h = { k1 => 'bar' }                        
puts h.has_key? k1 # -> true
puts h.has_key? k2 # -> still false ?? :·(