AJAX to change session data

Hi,
I am trying to figure out how to implement AJAX to set a session
variable when a user selects from a dropdown. I have not used AJAX
before.

My dropdown code is:
<%= collection_select(
          :question,
           :subject_id,
           Subject.order(:title),
           :id,
           :title,
           :prompt => "Select a Subject") %>

My coffeescript is
jQuery ->
  books = $('#question_book_id').html()
  $('#question_subject_id').change ->
    alert("Set the session[subject] now!")
    subject = $('#question_subject_id :selected').text()
    escaped_subject = subject.replace(/([
#;&,.+*~\':"!^$[\]()=>|\/@])/g, '\\$1')
    options =
$(books).filter("optgroup[label='#{escaped_subject}']").html()

I assume I create a method in users_controller to change the session
variable. My question is how do I call it in the jquery code?

This seems to work...

Routes:
match '/users/update_subject', :to => 'users#update_subject', :as =>
:update_subject

jQuery:
        ...
  $.ajax(
         url: '/users/update_subject',
         data: "subject_title=" + subject)

Controller:
def update_subject
    session[:subject] = params[:subject_title]
end

Can anyone tell me if it is OK to use a GET request (default) for this
or do I need to specify a POST, as I am only using this to set a
session variable and not hitting the database?

Thanks!