Answering the question #1:
I don't know the details of what's your overall goal, but if you'd like to have such URLs that are based on records from the database - then yes: it's a good approach.
Answering question #2 (regarding slugs):
I'd rather use something much more elaborate and well tested like: https://github.com/norman/friendly_id
My 50 cents about some other things:
Is this one of your first projects in Ruby/Rails? If so - congratulations! :) I'm asking because I noticed that you're using camel case here and there...
Also:
user_id = url.id.to_i
Why do you call this #to_i method here? Did you set up this id as a string or something?
Hope this helps