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

hash3.rb

class Key
  attr_reader :value

  def initialize(value)
    @value = value
  end

  def eql?(other_key)
    value == other_key.value 
  end

  def hash
    value.hash
  end 
end

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

h = { k1 => 'bar' }
puts h.has_key? k1 # -> true
puts h.has_key? k2 # -> true