TYPO3 extension - foreign domain class cannot be identified by property mapper

Question

I'm having a form in the frontend which creates an object of the model Question. Each question needs to be assigned one Category, which is selected in the form (via <f:form.select>) and then submitted like so:

tx_qa_questionform[question][category] = 1

basically submitting the Category's UID. After submitting the form I get an exception though:

#1297759968 TYPO3\CMS\Extbase\Property\Exception
Exception while property mapping at property path "category": Could not find a suitable type converter for "Category" because no such class or interface exists.

So it seems that the PropertyMapper can't identify the domain Category although being defined within the extension. Shouldn't this happen by itself? Or is there any configuration I missed to set up?

Model/Question.php

namespace Vendor\QA\Domain\Model;

class Question extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity {

    /**
     * @var Category $category
     */
    protected $category;

    /**
     * @var string $name
     */
    protected $name = '';

    /**
     * @var string $mail
     */
    protected $mail = '';

    /**
     * @var string $content
     */
    protected $content = '';

    public function __construct(Category $category, string $name, string $mail, string $content) {
        $this->setCategory($category);
        $this->setName($name);
        $this->setMail($mail);
        $this->setContent($content);
    }

    public function setCategory(Category $category): void {
        $this->category = $category;
    }

    public function getCategory(): Category {
        return $this->category;
    }

    public function setName(string $name): void {
        $this->name = $name;
    }

    public function getName(): string {
        return $this->name;
    }

    public function setMail(string $mail): void {
        $this->mail = $mail;
    }

    public function getMail(): string {
        return $this->mail;
    }

    public function setContent(string $content): void {
        $this->content = $content;
    }

    public function getContent(): string {
        return $this->content;
    }
}

Model/Category.php

namespace Vendor\QA\Domain\Model;

class Category extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity {

    /**
     * @var string $name
     */
    protected $name;

    public function __construct(string $name) {
        $this->setName($name);
    }

    public function setName(string $name): void {
        $this->name = $name;
    }

    public function getName(): string {
        return $this->name;
    }
}

Controller/FrontendController.php

namespace Vendor\QA\Controller;

use Vendor\QA\Domain\Model\Question;
use Vendor\QA\Domain\Repository\QuestionRepository;
use Vendor\QA\Domain\Repository\CategoryRepository;

class FrontendController extends \TYPO3\CMS\Extbase\Mvc\Controller\ActionController {

    private $questionRepository;
    private $categoryRepository;

    public function injectQuestionRepository(QuestionRepository $questionRepository) {
        $this->questionRepository = $questionRepository;
    }

    public function injectCategoryRepository(CategoryRepository $categoryRepository) {
        $this->categoryRepository = $categoryRepository;
    }

    public function formAction() {
        $categories = $this->categoryRepository->findAll();
        $this->view->assign('categories', $categories);
        $this->view->assign('newQuestion', null);
    }

    public function createQuestionAction(Question $question) {
        $this->questionRepository->add($question);
    }
}

Answer 1

You need @phpdoc annotations in your controller action functions. See for example the showAction here: https://docs.typo3.org/m/typo3/book-extbasefluid/master/en-us/7-Controllers/1-Creating-Controllers-and-Actions.html#flow-pattern-display-a-single-domain-object

After changing phpdoc annotations, clear the system caches.