symfony 2 cant save to database correctly - php

Question

To be short
my entity have this:
/**
* #ORM\ManyToOne(targetEntity="Classes", inversedBy="")
* #ORM\JoinColumn(name="class_id", referencedColumnName="id")
*/
protected $class;
i have no inverse relation, because i don't need it.
and i try to do this:
$class = $this->getDoctrine()->getRepository('TomasRClassBundle:Classes')->find($_POST['fighter']['class']);
$item = new Fighter();
$item->setName($_POST['fighter']['name']);
$item->setClass($class);
//if i var_dump here $item->getClass(), its there, class where set correctly
$item->setBelongsTo(1);
$em = $this->getDoctrine()->getManager();
$em->persist($item);
$em->flush();
and after that in database, in class_id i have null.. however if I edit it with form, this is form field:
$builder->add('class', 'entity', array('class' => 'TomasRClassBundle:Classes','property' => 'name'));
and after i submit the form i do:
$item = $this->getDoctrine()->getRepository('TomasRClassBundle:Fighter')->find($id);
$type = new FighterForm();
$form = $this->createForm($type, $item);
$form->handleRequest($request);
if ($form->isValid()) {
$em = $this->getDoctrine()->getManager();
$em->persist($item);
$em->flush();
//return $this->redirect($this->generateUrl('admin_fighter_list'));
}
and now the class_id field in db is set correctly.
i could hard code my solution, but i wanna know what i am doing wrong?
Thanks in advance, i know it might be a bit hard to read, sorry for that :)