select_tag - observe field

I'm trying to do an ajax call when the select (drop down) gets changed.
What's the best way to do this?

I wrote up something similar here:

http://lindsaar.net/2008/2/25/javascript-field-monitor

This is a javascript file that monitors every input and select on a
form and highlights it if it changes.

You can replace the highlight code with an AJAX call if you want.

Regards

Mikel

Here's how to put an observer on a select box:

VIEW:
<%= select_tag :card_type, %{<option value="">Please Select...</option>}