How to limit access to a certain mountable engine

initializer.rb

class CanAccessSomeGem
  def self.matches?(request)
    current_user = request.env['warden'].user
    return false if current_user.blank?
    return < some condition on current_user >
  end
end

routes.rb

  namespace :admin do
    constraints CanAccessSomeGem do
      mount SomeGem::Engine => "/somegem"
    end
  end