I'm not familiar with Fast_Seeder, but I am with friendly-id. Their docs say the following:
# If you're adding FriendlyId to an existing app and need
# to generate slugs for existing users, do this from the
# console, runner, or add a Rake task:
User.find_each(&:save)
It would obviously be more efficient to get friendly-id playing nice in the first place, but barring that you can add that the next line. You basically just need to tap validation. I bet .find_each(&:valid?)
might work too. This leads me to wonder if FastSeeder is creating these records without hitting your validations.
EDIT: Yup, I just dug through their source and they are creating straight through the database. You'll probably need to go the route I outlined above.