Drupal Ajax Forms - drupal

Question

I have a form in Drupal that calls an external database in Netezza. Retrieve this data from Netezza lasts about 10 seconds. Then, based on that information I have to build a select control to let the user choose from a list of categories. When the user chooses a category I do another expensive call to Netezza to retrieve more information.
The problem is that for the second interaction (when the user chose a category) the form is reprocessed and therefore doing 2 expensive calls to Netezza, not one as anyone would expect or desire.
Do you know a workaround for this situation? Is there a way to do an ajax call using the Drupal Ajax Framework without rebuilding the entire form?
Thanks.
PD: Reading documentation about the Ajax Framework I guess a solution could be using another path specifiying #ajax['path'], but haven´t fully tested that behavior and will be thankful if you share your experience.
PD2: I would prefer a workaround based on the Drupal Ajax Framework, not in a caching mechanism.

Answer 1

I'd highly recommend you to have a look into Drupal Examples, specially the module called ajax_example.
this is a fast sample code, might not be running, but just to give you the idea
function expensive_form($form, &$form_state) {
$form['category'] = array(
'#title' => t('Cateogry'),
'#type' => 'select',
'#options' => first_expensive_operation(),
'#ajax' => array(
'callback' => 'choose_category_callback',
'wrapper' => 'ajax-div',
// 'method' defaults to replaceWith, but valid values also include
// append, prepend, before and after.
// 'method' => 'replaceWith',
// 'effect' defaults to none. Other valid values are 'fade' and 'slide'.
'effect' => 'slide',
// 'speed' defaults to 'slow'. You can also use 'fast'
// or a number of milliseconds for the animation to last.
// 'speed' => 'slow',
),
);
$form['ajax_fieldset'] = array(
'#title' => t("Ajax Fields"),
// The prefix/suffix provide the div that we're replacing, named by
// #ajax['wrapper'] above.
'#prefix' => '<div id="ajax-div">',
'#suffix' => '</div>',
'#type' => 'fieldset',
'#description' => t('This is where we get automatically updated something'),
);
// this will only be executed on the second run of the form
// when the category is set.
if (isset($form_state['values']['category'])) {
$form['ajax_fieldset']['something'] = array(
'#title' => t('Somethings'),
'#type' => 'select',
'#options' => second_expensive_operation($form_state['values']['category']),
);
}
$form['submit'] = array(
'#type' => 'submit',
'#value' => t('Submit'),
);
return $form;
}
/**
* Callback element needs only select the portion of the form to be updated.
* Since #ajax['callback'] return can be HTML or a renderable
* array, we can just return a piece of the form.
*/
function choose_category_callback($form, $form_state) {
return $form['ajax_fieldset'];
}