Ruby on Rails - Can I modify data before it is saved?

Quick example: A user enters a username into a form, and I need to make that text username before storing it in the app's database, thereby making it permanently lowercase.

Where would I put this code, and how would I access the data to be lowercased?

Thanks.

you should overwrite the attribute writer:

class User < ActiveRecord::Base
  def username=(val)
    write_attribute(:username, val.downcase)
  end
end

You could use one of ActiveRecords callbacks in your User model, eg something like that:

before_save { |user| user.username = user.username.downcase }

In your User model (models/user.rb), take advantage of ActiveRecord callbacks:

before_save do
  self.username = self.username.downcase
end