how to pass form values within an ajax call?

hi guys,
  I am now rewriting an existing app (rails 2.3.8) with rails 3.1.x.

As observe_field() is no longer the way to go with rails 3 when ajax
is used, I am trying to rewrite a piece of functionality for when a
select box (drop down) with a div id of 'category' is clicked
(onChange),
-a call is made to the sub_categories controller with the category ID
(from the selected option in the category select box)
-the sub categories belonging to the selected category ID will be
returned and a another select box is to be generated in the div with
id of 'sub_category'.

I'm looking at http://www.simonecarletti.com/blog/2010/06/unobtrusive-javascript-in-rails-3/
and I have a question now.

How do I go about passing a variable value to the javascript that
performs the asynchronous call?

For example, i have :

// Append the function to the "document ready" chain
jQuery(function($) {
  // when the #search field changes
  $("#category").change(function() {
    // make a POST call and replace the content
    $.post(<%= category_sub_categories_path %>, function(data) {
      $("#results").html(data);
    });
  });
})

-how do I pass the category id (which is a form value) to the
category_sub_categories_path??
-would the category_id value be valid in the jQuery call?

This has nothing to do with Rails, this is a strictly jQuery question.
Take a look at the $.post documentation[1]

Cheers.
[1]http://api.jquery.com/jQuery.post/

1. The following javascript is being defined in myapp/public/
javascripts/application.js

jQuery(function($) {
  // when the #search field changes
  $("#category").change(function() {
    // make a POST call and replace the content
    $.post(<%= category_sub_categories_path %>, function(data) {
      $("#results").html(data);
    });
  });
})

2. this javascript is being applied to the myapp/app/views/parts/
_form.html.erb partial template

My question is, if there is a rails variable, @categories in the
partial template, I need to pass the @categories.id value to the
javascript from there.
Would this work?

If somebody understands this and could comment, kindly share your
input.
Thank you