After looking at your code, I think it is possible to make it work, there are a few things you will need to fix. I will edit this answer in a few steps.. Also make a backup/commit of your code before you start changing it.
1) In your MAIN entity (add cascade persist)
/**
* @var Sub
* @ORM\OneToMany(targetEntity="Sub", mappedBy="main", cascade={"persist"})
*/
protected $sub;
2) SUB entity:
Remove protected $mainId; and it's annotation.
Remove cascade={"persist"} from ManyToOne
3) Look at your controller action.
$sub = new Sub();
$sub->setMain($main);
$main->getSub()->add($sub);
Pay attention to the setMain() method. You do not want to do this in controller, but automatically in entity. And also you should add to collection manually, but make a method for it. So you will only have this:
$sub = new Sub();
$main->addSub($sub);
4) In MAIN entity add (you might need to import Sub):
public function addSub(Sub $sub) {
$sub->setMain($this);
$this->sub->add($sub);
return $this;
}
You should also add other methods like removeSub(), removeSub(), getSub(). getSub() returns collection, while the first two will return $this.
5) Controller
Do not persist Sub, but Main. (Doctrine will cascade persistance to Sub)
$em->persist($main);
6) You will need to add 'by_reference' option to sub collection inside you Main Form Type.
->add('sub', 'collection', array('type' => new SubType(), 'by_reference' => false));
This will call the actual addSub() method and not call the add method directly.
7) I do not know why you make a new Main entity below.
$em = $this->getDoctrine()->getManager();
$entity = $em->getRepository('ItemBundle:Main')
->findOneByPono($pono);
$cano = $entity->getCano();
$bano = $entity->getBano();
$main = new Main();
$main->setPono($pono);
$main->setCano($cano);
$main->setBano($bano);
Try to change to:
$em = $this->getDoctrine()->getManager();
$main = $em->getRepository('ItemBundle:Main')
->findOneByPono($pono);
You probably should define Pono as unique.