Why I use UUIDs for primary/foreign keys

[eluser]sl3dg3hamm3r[/eluser]
Interesting post here. I often thought of using UUIDs as well, simply to have an additional security-layer for free. Well, layer might be a wrong, or at least too strong expression, but not beeing able to guess any IDs is quite a plus. But of course, this alone makes a web-app not secure per se.

By the way: WAFs (Web Application Firewalls) also make links unreadable by encrypting them, a similar effect.

One thing I didn't like from the above posted link though:

If you want to create polymorphic associates than UUIDs are going to make that a lot easier for you in CakePHP. Just create a table called 'comments' and give it a belongsTo of [Photo, Post, User]. But instead of creating a foreign key field for each model, you get away with just creating a single foreign_id field and setting that up as the foreign key for all the belongsTo associations. Since the UUID the foreign_id points to is unique across tables, you don't even have to track what model / table it actually belongs to. This avoids a more complex query and leads to improved performance for the setup. (You might still want to populate Comment.model just so you know where the link goes to without doing the actual lookup).

This might be convenient, but you can't keep up referential integrity that way, or do I miss something?