SonataAdminBundle - check changes in `preUpdate` hook

This question is a bit similar to this one

Your solution is just to compare the field of the old object with the new one and see where it differs.

So for example:

public function preUpdate($newObject)
{
    $em = $this->getModelManager()->getEntityManager($this->getClass());
    $originalObject = $em->getUnitOfWork()->getOriginalEntityData($newObject);

    if ($newObject->getSomeField() !== $originalObject['fieldName']) {
        // Field has been changed
    }
}