how can i make two form input are related cakephp - cakephp

Question

I would like to make this in my add.ctp.
when user choose the department, on the file form field, only shows the file with the same department that they choose
in my add.ctp
<div class="form-group">
<?php echo $this->Form->input('department', array('class' => 'form-control', 'placeholder' => 'Department', 'options' => array(
'Administrator' => 'Administrator',
'Multimedia' => 'Multimedia',
'Treasurer' => 'Treasurer',
'Marketing' => 'Marketing',
),
'empty' => '(Choose Department)',));?>
</div>
<div class="form-group">
<?php echo $this->Form->input('fail_id', array('class' => 'form-control', 'label' => 'File','placeholder' => 'File Id', 'empty' => '(Choose File)'));?>
</div>
in my controller
public function add() {
if ($this->request->is('post')) {
$this->Borrow->create();
$this->request->data['Borrow']['user_id']= $this->Auth->user('id');
if ($this->Borrow->save($this->request->data)) {
$this->Session->setFlash(__('The borrow has been saved.'), 'default', array('class' => 'alert alert-success'));
return $this->redirect(array('action' => 'index'));
} else {
$this->Session->setFlash(__('The borrow could not be saved. Please, try again.'), 'default', array('class' => 'alert alert-danger'));
}
}
$users = $this->Borrow->User->find('list');
$fails = $this->Borrow->Fail->find('list');
$fails = $this->Borrow->Fail->find('list');
$this->set(compact('users', 'fails', 'fails'));
}
thanks for the kindness help.

Answer 1

You have to use ajax post. after user select a department you should send an ajax post with the value of department and get the related files and update second combobox.