How to make doctrine autofill generated values to model?

I have model that allows only to read property CreatedAt. This field is generated by SQL.

/**
   * @ORMId
   * @ORMColumn(type="integer")
   * @ORMGeneratedValue
   */
  private $Id;

...
/**
   * @ORMColumn(type="datetime", insertable=false, updatable=false)
   */
  private $CreatedAt;

It works fine exept 1 case: I can’t get that field upon creation:

$newObject = NewClassName();
$this->entityManager->persist($newObject);
$this->entityManager->flush();

$newObject->getId(); \ = 1234 (some generated integer)
$newObject->getCreatedAt(); \ = null

So question is: how to meke doctrine to autofill this field upon creation like it happens to $Id field

I’ve tried adding @GeneratedValue – does not work;

For now I use detach and getting it from DB after creation:

$newObject = NewClassName();
$this->entityManager->persist($newObject);
$this->entityManager->flush();
if ($newObject->getId()) {
      $this->entityManager->detach($newObject);
      $newObject = $this->entityManager->getRepository(NewClassName::class)->find($newObject->getId());


$newObject->getCreated(); \ = DateTime object

But I guess there is a better way. Thanks forahead!