Symfony Doctrine update many to one - symfony

Question

I have two entities:
1)
/**
* Post
*
* #ORM\Table(name="article")
* #ORM\Entity(repositoryClass="AppBundle\Repository\PostRepository")
*/
class Post
{
/**
* #var int
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
...
/**
* #var string
*
* #ORM\ManyToOne(targetEntity="AppBundle\Entity\Image", cascade={"persist"}, fetch="EAGER")
* #ORM\JoinColumn(name="image", referencedColumnName="id", nullable=true)
*/
private $image;
}
and 2)
/**
* Image
*
* #ORM\Table(name="image")
* #ORM\Entity(repositoryClass="AppBundle\Repository\ImageRepository")
*/
class Image
{
/**
* #var int
*
* #ORM\Column(name="id", type="string", length=40)
* #ORM\Id
*/
private $id;
/**
* #var string
*
* #ORM\Column(name="small", type="blob", nullable=true)
*/
private $small;
/**
* #var string
*
* #ORM\Column(name="medium", type="blob", nullable=true)
*/
private $medium;
/**
* #var string
*
* #ORM\Column(name="large", type="blob", nullable=true)
*/
private $large;
...
}
Both entities have getters and setters.
The controller sets the post to update
$requestFile = $request->files->get('image');
$em = $this->getDoctrine()->getManager();
$record = $em->getRepository('AppBundle:Post')->find($id);
if (!$record) {
throw $this->createNotFoundException(
'No product found for id '.$record
);
}
$record->setTitle($request->request->get('title'));
$record->constructAlias($request->request->get('title'));
...
if ($request->files->get('image')) {
// check if image already exists
$imgGeneratedId = sha1_file($request->files->get('image')->getPathName());
$imageRepo = $this->getDoctrine()->getRepository('AppBundle:Image');
$isPresent = $imageRepo->find($imgGeneratedId);
$image = $isPresent;
if (!$isPresent) {
$image = new Image();
$image->setId($imgGeneratedId);
$image->setLarge(file_get_contents($request->files->get('image')->getPathName()));
$mediumImage =
...
$em->persist($image);
}
$record->setImage($em->getReference('AppBundle:Image', $image->getId()));
}
$em->persist($record);
$em->flush();
Please ignore any syntax errors or other kinds; there are none.
The problem is that even if image already exists it is trying to create one which causes error in database. So, in order to avoid this I used getReference, but this causes other issues.
So, how can I force doctrine, not to insert an image if already exists and just set the article image with the image id?
Thanks.