Querying database with select2 based on selection from previous select2 - ajax

Question

I'm working on a symfony project (using Doctrine as well) and I'd like to implement a staggered search on one of the pages, per example:
User searches for an author in the first select2 box (which is pulling data from DB via Ajax), and once an item is selected, there is a second select2 box called title, which I would like to display only titles belonging to the selected author.
Here's the controller side code (both Ajax and controller) for the initial box. Any ideas how I could construct the query for the second select2?
The part that related to the initial select2 that queries the DB for the results and autosuggested items:
public function searchAjaxAuthorAction()
{
$em = $this->getDoctrine()->getManager();
$term = $this->get('request')->query->get('term');
$limit = $this->get('request')->query->get('page_limit', 1);
$rep = $em->getRepository('StephenPersianArtBundle:Main');
if($term){
$entities = $rep->createQueryBuilder('m')
->where('m.orig_author LIKE ?1')
->orderBy('m.orig_author', 'ASC')
->setParameter('1','%'.$term.'%')
->getQuery();
}else{
$entities = $rep->createQueryBuilder('m')
->groupBy('m.orig_author')
->getQuery();
}
$entities = $entities->execute();
$resultset = array();
foreach($entities as $entity){
if($entity->getOrigAuthor()){
$resultset[] = array(
//'id' => $entity->getId(),
'id' => $entity->getOrigAuthor(),
'text' => $entity->getOrigAuthor()
);
}
}
$return = json_encode($resultset);//jscon encode the array
return new Response($return,200,array('Content-Type'=>'application/json'));
}
There's also another part related to this that basically loads data into a table based on the item selected in the select2, but I don't consider this relevant for the issue I'm having as all this is happening before the final query is completed.
Any help will be much appreciated!