AJAX Error Drupal-7 - ajax

Question

I am creating a module in which there is a dependency between combo boxes. There are three combo boxes: country, state, and district. If I select India in the country combo box, then the state combo box will contain all states of India, and the same in the case of district.
I have done this by using AJAX. It works properly in add form, but when I am going to update any one, the following error occurs:
An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /drupal/?q=system/ajax
StatusText: Service unavailable (with message)
ResponseText: PDOException: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'ajax' in 'where clause': SELECT district_name, country_id,state_id FROM {ajax_district} where id = ajax; Array
(
)
in edit_district_form() (line 291 of /home/mansmu/www/drupal/sites/default/modules/ajaxtest/ajaxtest.district.inc).
My code is:
////////////////////////////////////////////////////////
///////// FUNCTION FOR EDIT
////////////////////////////////////////////////////////
function edit_district_form($form, &$form_state) {
$request_url = request_uri();
// $request_id to get id of edit district.
$request_id = drupal_substr(strrchr($request_url, '/'), 1);
//// FOR country
$rows = array();
$result = db_query("SELECT id,country_name from {ajax_country} order by country_name");
while ($data = $result->fetchObject()) {
$id = $data->id;
$rows[$id] = $data->country_name;
}
//// FOR state
$state = array();
$result = db_query("SELECT id,state_name from {ajax_state} order by state_name");
while ($data = $result->fetchObject()) {
$id = $data->id;
$state[$id] = $data->state_name;
}
// $district_name varible to get name from database for a requested id.
$district_name = db_query("SELECT district_name, country_id,state_id FROM {ajax_district} where id = ".$request_id);
// Loop For show vehicle district name in text field to be update.
foreach ($district_name as $district) {*/
$form['values']['edit_country_name'] = array(
'#type' => 'select',
// '#default_value' => "{$district->country_id}",
'#required' => TRUE,
'#options' => $rows,
'#ajax' => array(
'effect' => 'fade',
'progress' => array('type' => 'none'),
'callback' => 'state_callback_edit',
'wrapper' => 'replace_edit_state',
),
);
$form['values']['edit_state_name'] = array(
'#type' => 'select',
// '#default_value' => "{$district->state_id}",
'#required' => TRUE,
'#options' => array(),
// The prefix/suffix provide the div that we're replacing, named by #ajax['wrapper'] above.
'#prefix' => '',
'#suffix' => '',
);
$form['values']['edit_district_name'] = array(
'#type' => 'textfield',
'#size' => 30,
//'#default_value' => "{$district->district_name}",
'#maxlength' => 80,
'#required' => TRUE,
);
}
$form['edit_district_submit'] = array(
'#type' => 'submit',
'#value' => t('Update'),
);
$form['actions']['cancel'] = array(
'#markup' => l(t('Cancel'), 'district'),
);
return $form;
}
////////////////////////////////////////////////////////
///////// FUNCTION FOR AJAX CALL BACK
////////////////////////////////////////////////////////
function state_callback_edit($form, &$form_state) {
// An AJAX request calls the form builder function for every change.
// We can change how we build the form based on $form_state.
if (!empty($form_state['values']['edit_country_name'])) {
$country_id = $form_state['values']['edit_country_name'];
$rows1 = array();
$result = db_query("SELECT id, state_name from {ajax_state} where country_id = $country_id order by state_name");
while ($data = $result->fetchObject()) {
$id = $data->id;
$rows1[$id] = $data->state_name;
}
$form['edit_state_name']['#options'] = $rows1;
}
return $form['values']['edit_state_name'];
}

Answer 1

Your line of code here:
$request_id = drupal_substr(strrchr($request_url, '/'), 1);
is returning the string 'ajax', making your third SQL statement look like this:
SELECT district_name, country_id,state_id FROM {ajax_district} where id = ajax;
Obviously 'ajax' is wrong there, I guess you want a value from the URL? You could use the arg() function to extract this so you don't need to run drupal_substr etc.
If your path is ajax/12 then arg(0) will return 'ajax', arg(1) will return 12 and so on.
Hope that helps
UPDATE
If you need to save the request ID in the form do something like this in your form function:
$form['request_id'] = array('#type' => 'value', '#value' => $request_id);
Then when you get into your ajax callback you can either get it from $form_state['values']['request_id'] or $form['request_id']['#value']