Rails equivalent of globally unique / object id system

I have a php app and there there is some consideration about porting it to Rails.

One piece where it is unclear whether there is a prebuilt equivalent to a specific part. It has a table with a unique identifying system for objects. Not ALL objects participate in this system, just objects that are considered 'interesting'.

A simplified example:

user - uses system
id user_name
1   joe
2   frank

image - uses system
id url
1   /img/123.jpg
2   /img/234.jpg

comment - uses system
id comment 
1  what are you doing?
2  this is great

global_identification
id  object_type object_id
1   user        1
2   comment     1
3   image       1
4   user        2
5   image       2
6   comment     2

globals_lists
1   list_id     global_id
2   1           3            
3   1           6

This has simplified a lot of elements esp for list creations having only to have开发者_开发技巧 a global_identifiter rather than an object and an id. It's a php web app so essentially single-threaded.

This is essentially hand-rolled and am curious whether there is a rails equivalent of something like this? Also, talking with another developer, I was curious whether there is a name for this - it's kinda like a half-baked GUID system?

There is no in-language support for guid.

Here is a an articles that talks about guid: http://amthekkel.blogspot.com/2009/02/ruby-on-rails-how-to-use-guid-for-use.html