ruby hash as key to a hash

Question

Came across the following weird behaviour in ruby 1.8.6, in 1.8.7 it seems to be working correctly. Does anyone know what would have caused this?

h = {}
key_1 = {1 => 2}
key_2 = {1 => 2}
h[key_1] = 3
p key_1 == key_2 # => true
p h.has_key?(key_2) # => expect true, get false, wtf?

I had thought that it would be caused by the implementation of the hash method on the Hash class.

p [key_1.hash, key_2.hash] # => [537787070, 537787060] (different)

but even if I override the hash method of Hash

class Hash
  def hash
    return self.keys.hash + self.values.hash
  end
end

p [key_1.hash, key_2.hash] # => [8,8] (same
p h.has_key?(key_2)        # => false

codepad link to online ruby 1.8.6 interpreter results: http://codepad.org/7nCYMP4w

Answer 1

This is fixed in 1.8.7+ but you can monkey patch 1.8.6 to do it right, too ex: https://github.com/rdp/sane/blob/master/lib/sane/hash_hashes.rb

Answer 2

The answer is because in Ruby 1.8.6 the hash coding algorithm was broken for hash keys.

http://paulbarry.com/articles/2009/09/14/why-rails-3-will-require-ruby-1-8-7

Edit: Here is an example that shows that ruby does not call .hash internally:

 class Hash
    def hash
       raise
    end
 end

 {1=>1}.hash
 RuntimeError: 
from (irb):12:in `hash'
from (irb):17

 h = {1=>2}
 {1=>2}
 h[1]
 2

Ruby 1.8.6 is broken in this respect, and if there were a pure Ruby way to do it (such as opening Hash, people would do it. It was fixed in 1.8.7