user.rb

class User < ActiveRecord::Base
  acts_as_authentic
  
  validates_format_of :username, :with => /\A[\w-]+\Z/
  validate :username_is_not_a_route # prevents usernames that are existing routes
  
  protected
    
    def username_is_not_a_route
      path = ActionController::Routing::Routes.recognize_path("/#{username}", :method => :get) rescue nil
      errors.add(:username, "conflicts with existing path (/#{username})") if path && !path[:username]
    end
    
end