cake php, how to populate a drop down selection based on another drop down selection?

I have a table something like this:

id  test   attr  dim
1   test1  a1    d1
2   test2  a2    d2
3   test3  a1    d3
4   test3  a1    d2

I am creating a drop down field that shows the 开发者_运维技巧attr as having only two options a1 and a2. Based on a selection of either a1 or a2 i would like to do is something like:

if (a1 is selected){$a == a1} else {$a == a2}
$test = mysql_query("SELECT dim FROM xxx WHERE attr = $a")

and fill up the second drop down with the results from that query that could be d1, d2 or d3

and so on, on multiple drop down lists if I need to.

I know how to do this in regular php but not sure how to do it in CakePHP.

Any ideas?

Edit: I'm not sure if i need to use any ajax, this should be working without it

You can do with help of ajax->observefield

eg:

$options = array('url' => 'getcounties', 'update' => 'ExampleCountyI
$ajax->observeField('ExampleStateId', $options);

UPDATED

observeField is not more available in CakePHP js helper here it's equivalent way to the same for newer CakePHP version >= 2.x