update an entity with an ID column which is a relation instead of a normal field

<?php

namespace Roompot\TRSBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Samson\Bundle\TRSBundle\Entity\Registrar;

/**
 * @ORM\Entity
 */
class RegistrarDepartmentMapping
{
    /**
     * @ORM\ManyToOne(targetEntity="RoompotRegistrar")
     * @ORM\JoinColumn(referencedColumnName="person_id")
     * @ORM\Id
     */
    private $registrar;

    /**
     * @ORM\ManyToOne(targetEntity="Department")
     * @ORM\Id
     */
    private $department;

    /**
     * @ORM\Column(type="boolean")
     */
    private $head = false;

    public function getRegistrar()
    {
        return $this->registrar;
    }

    public function setRegistrar(Registrar $registrar)
    {
        if (null !== $this->registrar) {
            throw new \RuntimeException('Cannot change registrar! Remove this entity and create a new one');
        }
        $this->registrar = $registrar;
    }

    public function getDepartment()
    {
        if (null !== $this->registrar) {
            throw new \RuntimeException('Cannot change department! Remove this entity and create a new one');
        }
        return $this->department;
    }

    public function setDepartment(Department $department)
    {
        $this->department = $department;
    }

    public function isHead()
    {
        return $this->head;
    }

    public function setHead($head)
    {
        $this->head = $head;
    }
}

<?php

namespace Roompot\TRSBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Samson\Bundle\TRSBundle\Entity\Registrar;

/**
 * @ORM\Entity
 */
class RoompotRegistrar extends Registrar
{
    [...]
}

<?php

namespace Samson\Bundle\TRSBundle\Entity;

use Samson\Bundle\AddressBookBundle\Entity\Person;
use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity 
 * @ORM\InheritanceType("SINGLE_TABLE")
 * @ORM\DiscriminatorColumn(name="discr", type="string")
 */
abstract class Registrar
{
    /**
     * @ORM\Id
     * @ORM\OneToOne(targetEntity="Samson\Bundle\AddressBookBundle\Entity\Person", cascade={"persist"})
     * @ORM\JoinColumn(nullable=false, onDelete="CASCADE")
     */
    private $person;

    [...]
}

<?php

namespace Samson\Bundle\AddressBookBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity(repositoryClass="Samson\Bundle\AddressBookBundle\Entity\PersonRepository")
 */
class Person implements [...]
{
    /**
     * @ORM\Column(type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue
     */
    private $id;

    [...]
}