This isn’t necessarily a direct answer to your question, but have you looked at the Stringex plugin (http://github.com/rsl/stringex)? It’s a great way to auto-create slugs for your records.
You can just add something like the following to your model:
class Post < ActiveRecord::Base
acts_as_url :title
end
and it will auto-create slugs from your title and save it to the slug column.
It’s also really smart about the way it creates slugs. For example, a title of “10% off, today only” gets turned into “10-percent-off-today-only”.
Pretty slick!