Could not resolve type of column of class

I want to establish a one to many relashionship. One 'Uczen' should have one 'klasa' and one 'Klasa' should have many 'Uczen'. I am using Doctrine

index.php

    <?php 
require_once 'vendor/autoload.php';
use Doctrine\ORM\ORMException;
use Doctrine\ORM\ORMSetup;
use Doctrine\ORM\EntityManager;
use Doctrine\Common\Collections\ArrayCollection;
include 'src/Uczen.php';
include 'src/Klasa.php';

$config = ORMSetup::createAnnotationMetadataConfiguration(array(__DIR__  . '/src'));
$conn = array(
    'dbname' => 'zi2',
    'user' => 'root',
    'password' => 'user',
    'host' => 'localhost',
    'driver' => 'pdo_mysql',
    'charset' => 'utf8'
);
$em = EntityManager::create($conn, $config);

$k1 = new Klasa();
$k1->setId(1);
$k1->setNazwa("312B");
$k1->setRok(3);

$u1 = new Uczen();
$u1->setId(1);
$u1->setImie('Jan');
$u1->setNazwisko('Kowalski');

// tablice
$uczniowie1 = new ArrayCollection([$u1]); // uczniowie klasy k1

// Foreign keys
$k1->setUczniowie($uczniowie1);

$u1->setKlasa($k1);

$em->persist($k1);
$em->persist($u1);
$em->flush();

?>

Uczen.php

<?php 
use Doctrine\ORM\Mapping\Entity;
use Doctrine\ORM\Mapping\Id;
use Doctrine\ORM\Mapping\Column;
use Doctrine\ORM\Mapping\GeneratedValue;
use Doctrine\ORM\Mapping\ManyToOne;

/** 
 * @Entity 
 */
class Uczen
{
    /** 
     * @Id
     * @Column(type="integer") 
     * @GeneratedValue
     */
    protected $id_uczen;
    /** 
     * @Column(type="string") 
     */
    protected $imie;
    /** 
     * @Column(type="string") 
     */
    protected $nazwisko;
    /** 
     * @ManyToOne(targetEntity="Klasa", inversedBy="uczniowie")
     */
    protected $klasa;

    public function setId($id){
        $this->id_uczen = $id;
    }

    public function getId(){
        return $this->id_uczen;
    }

    public function setImie($imie){
        $this->imie = $imie;
    }

    public function getImie(){
        return $this->imie;
    }

    public function setNazwisko($nazwisko){
        $this->nazwisko = $nazwisko;
    }

    public function getNazwisko(){
        return $this->nazwisko;
    }

    public function setKlasa($klasa){
        $this->klasa = $klasa;
    }

    public function getKlasa(){
        return $this->klasa;
    }
}
?>

Klasa.php

<?php 
use Doctrine\ORM\Mapping\Entity;
use Doctrine\ORM\Mapping\Id;
use Doctrine\ORM\Mapping\Column;
use Doctrine\ORM\Mapping\GeneratedValue;
use Doctrine\ORM\Mapping\OneToMany;
use Doctrine\Common\Collections\ArrayCollection;

/** 
 * @Entity 
 */
class Klasa
{
    /** 
     * @Id
     * @Column(type="integer") 
     * @GeneratedValue
     */
    protected $id_klasa;
    /** 
     * @Column(type="string") 
     */
    protected $nazwa;
    /** 
     * @Column(type="integer") 
     */
    protected $rok;
    /** 
     * @OneToMany(targetEntity="Uczen",mappedBy="klasa")
     */
    protected $uczniowie;

    public function __construct(){
        $this->uczniowie = new ArrayCollection();
    }

    public function setId($id){
        $this->id_klasa = $id;
    }

    public function getId(){
        return $this->id_klasa;
    }

    public function setNazwa($nazwa){
        $this->nazwa = $nazwa;
    }

    public function getNazwa(){
        return $this->nazwa;
    }

    public function setRok($rok){
        $this->rok = $rok;
    }

    public function getRok(){
        return $this->rok;
    }

    public function setUczniowie($uczniowie){
        $this->uczniowie = $uczniowie;
    }

    public function getUczniowie(){
        return $this->uczniowie;
    }
}
?>

Error:

PHP Fatal error: Uncaught RuntimeException: Could not resolve type of column "id" of class "Klasa" in C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Utility\PersisterHelper.php:110 Stack trace: #0 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Persisters\Entity\BasicEntityPersister.php(711): Doctrine\ORM\Utility\PersisterHelper::getTypeOfColumn('id', Object(Doctrine\ORM\Mapping\ClassMetadata), Object(Doctrine\ORM\EntityManager)) #1 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Persisters\Entity\BasicEntityPersister.php(736): Doctrine\ORM\Persisters\Entity\BasicEntityPersister->prepareUpdateData(Object(Uczen), true) #2 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Persisters\Entity\BasicEntityPersister.php(270): Doctrine\ORM\Persisters\Entity\BasicEntityPersister->prepareInsertData(Object(Uczen)) #3 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\UnitOfWork.php(1144): Doctrine\ORM\Persisters\Entity\BasicEntityPersister->executeInserts() #4 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\UnitOfWork.php(430): Doctrine\ORM\UnitOfWork->executeInserts(Object(Doctrine\ORM\Mapping\ClassMetadata)) #5 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\EntityManager.php(403): Doctrine\ORM\UnitOfWork->commit(NULL) #6 C:\Users\sheil\Desktop\Temp\index.php(41): Doctrine\ORM\EntityManager->flush() #7 {main} thrown in C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Utility\PersisterHelper.php on line 110

Fatal error: Uncaught RuntimeException: Could not resolve type of column "id" of class "Klasa" in C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Utility\PersisterHelper.php:110 Stack trace: #0 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Persisters\Entity\BasicEntityPersister.php(711): Doctrine\ORM\Utility\PersisterHelper::getTypeOfColumn('id', Object(Doctrine\ORM\Mapping\ClassMetadata), Object(Doctrine\ORM\EntityManager)) #1 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Persisters\Entity\BasicEntityPersister.php(736): Doctrine\ORM\Persisters\Entity\BasicEntityPersister->prepareUpdateData(Object(Uczen), true) #2 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Persisters\Entity\BasicEntityPersister.php(270): Doctrine\ORM\Persisters\Entity\BasicEntityPersister->prepareInsertData(Object(Uczen)) #3 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\UnitOfWork.php(1144): Doctrine\ORM\Persisters\Entity\BasicEntityPersister->executeInserts() #4 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\UnitOfWork.php(430): Doctrine\ORM\UnitOfWork->executeInserts(Object(Doctrine\ORM\Mapping\ClassMetadata)) #5 C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\EntityManager.php(403): Doctrine\ORM\UnitOfWork->commit(NULL) #6 C:\Users\sheil\Desktop\Temp\index.php(41): Doctrine\ORM\EntityManager->flush() #7 {main} thrown in C:\Users\sheil\Desktop\Temp\vendor\doctrine\orm\lib\Doctrine\ORM\Utility\PersisterHelper.php on line 110

Solution

/** 
     * @ManyToOne(targetEntity="Klasa",inversedBy="uczniowie",cascade={"persist"})
     * @JoinColumn(name="klasa",nullable=false,referencedColumnName="id_klasa")
     */
    protected $klasa;