Symfony2 + OneToMany - read from database error - symfony

Question

I'm trying to learn read/write data to/from database, and I have a huge problem :
My entities looks :
Kategorie:
use Doctrine\Common\Collections\ArrayCollection;
/**
* #ORM\Entity
* #ORM\Table(name="kategorie")
*/
class Kategorie
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $idkategorii;
/**
* #ORM\Column(type="string", length=50)
*/
protected $nazwa;
/**
* #ORM\OneToMany(targetEntity="Ogloszenia", mappedBy="ogloszenia")
*/
protected $ogloszenia;
Ogloszenia:
use Doctrine\ORM\Mapping as ORM;
/**
* #ORM\Entity
* #ORM\Table(name="ogloszenia")
*/
class Ogloszenia
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #ORM\Column(type="string",length=100)
*/
protected $tytul;
/**
* #ORM\Column(type="string", length=120)
*/
protected $tytul_seo; //tytul bez polskich znaków etc.
/**
* #ORM\Column(type="text")
*/
protected $tresc; //tresc ogloszenia
/**
* #ORM\Column(type="string",length=50)
*/
protected $dodal; //imie osoby ktora dodala ogloszenie
/**
* #ORM\Column(type="string", length=50)
*/
protected $kontakt; //nr tel lub mail
/**
* #ORM\ManyToOne(targetEntity="Kategorie", inversedBy="kategoria")
* #ORM\JoinColumn(name="kategoria", referencedColumnName="idkategorii")
*/
protected $kategoria;
Now, in my controller i'm trying to read all values :
public function odczytajAction()
{
$id = 1;
$kategoria = $this->getDoctrine()
->getRepository('FrontendOgloszeniaBundle:Kategorie')
->find($id);
$ogl = $kategoria->getOgloszenia();
foreach($ogl as $o)
{
print_r($o);
}
return new Response('test odczytu');
}
And unfortunately symfony2 gives me an following error
Notice: Undefined index: ogloszenia in /home/sl4sh/public_html/Projekt1/vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php line 1574
500 Internal Server Error - ErrorException
So, please tell me what's wrong with mine code ?

Answer 1

You have invalid mappedBy annotation:
/**
* #ORM\OneToMany(targetEntity="Ogloszenia", mappedBy="kategoria")
*/
protected $ogloszenia;
And also inverse side:
/**
* #ORM\ManyToOne(targetEntity="Kategorie", inversedBy="ogloszenia")
* #ORM\JoinColumn(name="kategoria", referencedColumnName="idkategorii")
*/
protected $kategoria;