TYPO3 8 with extbase: remove FileReference - typo3

Question

This seems simple enough but I seem to be too stupid to do it. I have added a field to fe_users called "cv" for uploading a pdf file. In my FrontendUser Model it looks like this:
/**
* #var \TYPO3\CMS\Extbase\Domain\Model\FileReference
* #cascade remove
*/
protected $cv;
Uploading a file works like a charm, I used the script from https://github.com/helhum/upload_example in parts.
The only problem is deleting such a file, or rather, removing the connection between sys_file and the user. After submitting a form and checking a checkbox, I tried to do this:
$user->setCv(null);
$user->setEdited(new \DateTime());
$this->frontendUserRepository->update($user);
$persistenceManager->persistAll();
After a page reload if I take a look at the backend, the cv File is still attached to the user (but "edited" was correctly set to the current datetime). I do not understand this, how can I set the FileReference Value to null?

Answer 1

Make sure to check your Model. FileReferences are stored in an M:N relation. So you must use the folling declaration in your Model:
/**
* cv
*
* #var \TYPO3\CMS\Extbase\Persistence\ObjectStorage<\TYPO3\CMS\Extbase\Domain\Model\FileReference>
* #cascade remove
*/
protected $cv = null;
with an initialization in the __constructor:
public function __construct()
{
$this->cv = new \TYPO3\CMS\Extbase\Persistence\ObjectStorage();
}
Now you have your FileReference in an ObjectStorage. To clear all FileReferences you can set cv to new \TYPO3\CMS\Extbase\Persistence\ObjectStorage():
$user->removeCv(new \TYPO3\CMS\Extbase\Persistence\ObjectStorage());
$user->setEdited(new \DateTime());
$this->frontendUserRepository->update($user);
$persistenceManager->persistAll();