doctrine how to merge incomplete data - php

Question

I'm creating an REST api with Symfony 4.2 and Doctrine as ORM connected to a MySQL database. I receive data in JSON , deserialize them with JMSSerializer and use the doctrine merge function to attach it. How can I merge the entity I got from the JSON with an already existing entity, updating only the given fields in the JSON?
Event entity
<?php
namespace App\Entity;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use Doctrine\ORM\Mapping as ORM;
use JMS\Serializer\Annotation\Type;
/**
* Event
*
* #ORM\Table(name="Event", indexes={#ORM\Index(name="Organiser", columns={"Organiser"})})
* #ORM\Entity
*/
class Event
{
/**
* #var int
*
* #ORM\Column(name="ID", type="integer", nullable=false)
* #ORM\Id
* #ORM\GeneratedValue
* #Type("int")
*/
private $id;
/**
* #var \Invited
*
* #ORM\OneToMany(targetEntity="Invited",mappedBy="event",cascade={"persist","merge","remove"})
* #Type("ArrayCollection<App\Entity\Invited>")
*/
private $guests;
/**
* #var string
*
* #ORM\Column(name="Title", type="string", length=255, nullable=false)
* #Type("string")
*/
private $title;
/**
* #var \DateTime
*
* #ORM\Column(name="StartDate", type="datetime", nullable=false)
* #Type("DateTime")
*/
private $startdate;
/**
* #var \DateTime|null
*
* #ORM\Column(name="EndDate", type="datetime", nullable=true)
* #Type("DateTime")
*/
private $enddate;
/**
* #var string
*
* #ORM\Column(name="Place", type="string", length=255, nullable=false)
* #Type("string")
*/
private $place;
/**
* #var string|null
*
* #ORM\Column(name="ImagePath", type="string", length=255, nullable=true)
* #Type("string")
*/
private $imagepath;
/**
* #var \User
*
* #ORM\ManyToOne(targetEntity="User",cascade={"persist"})
* #ORM\JoinColumns({
* #ORM\JoinColumn(name="Organiser", referencedColumnName="ID")
* })
* #Type("App\Entity\User")
*/
private $organiser;
?>
Deserialisation
$json = $request->getContent();
$serializer = SerializerBuilder::create()->build();
$event = $serializer->deserialize($json,'App\Entity\Event','json');
Merging
$feedback = $this->em->merge($newEvent);
$this->em->flush();
Expected vs Actual results:
If I have an already existing and attached event like:
{
id:1,
title: 'oldTitle',
startDate: oldDate,
place: 'oldPlace',
...
}
And I receive :
{
id:1,
title: 'newTitle'
}
Expected result:
{
id:1,
title: 'newTitle',
startDate: oldDate,
place: 'oldPlace',
...
}
Actual result when looking to the database:
{
id:1,
title: 'newTitle',
startDate: null,
place: null,
...
}
How can I achieve that in a efficient way?