Call to undefined method getPosition() - Why and Fix - typo3

Question

I made an extbase extension with a class Appointment with a property expertises und another one subExpertises of the same type.
This is how they look like in the Appointment class (subExpertises is the same):
/**
* expertises
*
* #var \TYPO3\CMS\Extbase\Persistence\ObjectStorage<...\Domain\Model\Expertise>
*/
protected $expertises = NULL;
/**
* Adds an expertise
*
* #param ...\Domain\Model\Expertise $expertise
* #return void
*/
public function addExpertise(...\Domain\Model\Expertise $expertise) {
$this->expertises->attach($expertise);
}
I get an error when executing this code in my controller after editing the appointment in a fluid form:
/**
*
* #param \Domain\Model\Appointment $appointment
* #return void
*/
public function bookAction(\Domain\Model\Appointment $appointment) {
//empty all expertises of appointment - then fill them with the selected from lawyer
$appointment->setExpertises(new \TYPO3\CMS\Extbase\Persistence\ObjectStorage());
$appointment->setSubExpertises(new \TYPO3\CMS\Extbase\Persistence\ObjectStorage());
//add all checked expertises of lawyer to appointment
foreach ($appointment->getLawyer()->getExpertises() as $expertise) {
if ($expertise->getChecked()) {
$appointment->addExpertise($expertise);
}
foreach ($expertise->getSubExpertises() as $subExpertise) {
if ($subExpertise->getChecked()) {
$appointment->addSubExpertise($subExpertise);
}
}
}
$this->appointmentRepository->update($appointment);
}
This is the error:
Fatal error: Call to undefined method \Domain\Model\Expertise::getPosition() in /var/www/typo3_src/typo3_src-6.2.25/typo3/sysext/extbase/Classes/Persistence/Generic/Backend.php on line 453
Now it seems that TYPO3 thinks Expertise is of type ObjectStorage because it tries to call getPosition() but I have no clue why it does that and what I should change in order to successfully save my Appointment object with the new Expertises.
I tried debugging the appointment object, but I couldn't find the problem - it seems okay to me, it just shows that expertises und subExpertises have been modified.

Answer 1

Getter methods in Extbase aren't magic, you have to explicitly define them.
If you're dealing with a n:n-relation, you also need to initialize the Property as ObjectStorage in your model and configure it in the TCA.
/**
* Initialize all ObjectStorage properties.
*
* #return void
*/
protected function initStorageObjects() {
$this->yourProperty = new \TYPO3\CMS\Extbase\Persistence\ObjectStorage();
}