using route resource in the model

How would i go about using a mapped resource in my model?

eg.
# routes file
map.resources :questions

# model
def after_create
    notification = UserNotification.new(:user => current_user)
    notification.body = 'You posted a new <a href="' +
question_url(question.id) + '">question</a>'
    notification.save
end

comes up saying undefined method for question_url

Well, since url helpers are UI constructs, they really shouldn't be
used directly by models. They are available in controllers and views.

I'd humbly suggest that the best way to solve this is to have the
UserNotification have a reference to the question, and then use
question_url in UserNotificationsController and/or it's views.

This would properly keep the M-VC business logic-ui separation.

mmm ok thanks the help