Different hashes for the same object values in ruby

Question

I have a problem with "hash" method from ruby: http://apidock.com/ruby/v1_8_7_330/Hash/hash

I use this to compare two different objects with the same content to find collisions between data base objects and new elements with the same attributes.

It has been apparently working for a long time (more than one year) but it has suddenly stopped working in my dev computer.

If I try this in the rails console:

a = {:a => 'a', :b=>'b'}
b = {:a => 'a', :b=>'b'}
a.hash
b.hash

I get the same value for both a.hash and b.hash (-3820017043059270405) in the same console. The problem is, if I change to another console it returns a different value from the first console (-1865824882785682267).

Shouldn't it be returning the same hash ever?

Thanks in advance.

Edit: It worked good in production because of ree 1.8.7 version.

Answer 1

No, it should not!

Different Ruby implementations (like jRuby, Rubinius, MRI 1.8.x, MRI 1.9.x etc) are using different ways to generate hashes. For example, for some objects (like you own classes or Hash instances) runtime will assign uniq and random id while creating this object. If I am not wrong, MRI tight works with hashes based on memory addresses: http://rxr.whitequark.org/mri/source/gc.c?v=1.8.7-p370#2111

So you can not guarantee that every run of your code will use the same random values or the same memory addresses every time.

Also I suggest to use ruby-doc instead of apidock for Ruby internals: http://ruby-doc.org/core-2.0/Object.html#method-i-hash

The hash value for an object may not be identical across invocations or implementations of ruby. If you need a stable identifier across ruby invocations and implementations you will need to generate one with a custom method.

Hope it will help you!