Rails 4 user role constraint

role_constraint.rb

# app/constraints/role_constraint.rb
class RoleConstraint
  def initialize(*roles)
    @roles = roles.map { |r| r.to_s }
  end

  def matches?(request)
    @roles.include? request.env['warden'].user(:user).try(:role)
  end
end

route.rb

# config/route.rb

root :to => 'admin#index', :constraints => RoleConstraint.new(:admin) #matches this route when the current user is an admin
root :to => 'sites#index', :constraints => RoleConstraint.new(:user) #matches this route when the current user is an user
root :to => 'home#index' #matches this route when the above two matches don't pass

hello, I came to here from Google, and follow an old post:
http://minhajuddin.com/2011/10/24/how-to-change-the-rails-root-url-based-on-the-user-or-role

But I couldn't let this beautiful trick works.
Should I put role_constraint.rb in app/lib ?