Could not determine access type for property "filters".

project.php

<?php
# AppBundle/Entity/Category.php ================================================================================================
namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * Category
 *
 * @ORM\Table(name="categories")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\CategoryRepository")
 */
class Category
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="name", type="string", length=255)
     */
    private $name;

    /**
     * @ORM\ManyToMany(targetEntity="Filter")
     * @ORM\JoinTable(name="category_filters")
     */
    private $filters;

    public function __construct()
    {
        $this->filters = new ArrayCollection();
    }

    /**
     * Get id
     *
     * @return int
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set name
     *
     * @param string $name
     *
     * @return Category
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

    /**
     * Get name
     *
     * @return string
     */
    public function getName()
    {
        return $this->name;
    }

    public function addFilter(Filter $filter) {
        if ($this->filters->contains($filter)) {
            return;
        }
        $this->filters[] = $filter;

        return $this;
    }
    
    public function removeFilter(Filter $filter) {
        if (!$this->filters->contains($filter)) {
            return;
        }

        $this->filters->removeElement($filter);

        return $this;
    }

    public function getFilters()
    {
        return $this->filters;
    }
}

# AppBundle/Entity/Filter.php ==================================================================================================
namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * Filter
 *
 * @ORM\Table(name="filters")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\FilterRepository")
 */
class Filter
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="value", type="string", length=255)
     */
    private $value;


    /**
     * Get id
     *
     * @return int
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set value
     *
     * @param string $value
     *
     * @return Filter
     */
    public function setValue($value)
    {
        $this->value = $value;

        return $this;
    }

    /**
     * Get value
     *
     * @return string
     */
    public function getValue()
    {
        return $this->value;
    }
}

# AppBundle/Form/CategoryForm.php ===============================================================================================
namespace AppBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Bridge\Doctrine\Form\Type\EntityType;
use Doctrine\ORM\EntityRepository;

class CategoryType extends AbstractType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('name')
            ->add('filters', EntityType::class, [
                'class' => 'AppBundle:Filter',
                'placeholder' => ' ',
                'query_builder' => function(EntityRepository $er) {
                    return $er->createQueryBuilder('f');
                },
                'choice_label' => function($filters) {
                    return $filters->getValue();
                },
                'multiple' => false,
                'expanded' => false,
                // 'mapped' => false,
                // 'by_reference' => false,
            ])
        ;
    }

    /**
     * {@inheritdoc}
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\Category'
        ));
    }

    /**
     * {@inheritdoc}
     */
    public function getBlockPrefix()
    {
        return 'appbundle_category';
    }
}

# AppBundle/Form/FilterType.php =================================================================================================

namespace AppBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class FilterType extends AbstractType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('value')
        ;
    }

    /**
     * {@inheritdoc}
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\Filter'
        ));
    }

    /**
     * {@inheritdoc}
     */
    public function getBlockPrefix()
    {
        return 'appbundle_filter';
    }
}

# AppBundle/Controller/DefaultController.php ====================================================================================
namespace AppBundle\Controller;

use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Method;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;
use AppBundle\Entity\Category;
use AppBundle\Form\CategoryType;
use AppBundle\Entity\Filter;
use AppBundle\Form\FilterType;

class DefaultController extends Controller
{
    /**
     * @Route("/", name="homepage")
     */
    public function indexAction()
    {
        return $this->render('default/index.html.twig', []);
    }

    /**
     * @Route("/category", name="category_index")
     */
    public function categoryIndexAction()
    {
        $categories = $this->getDoctrine()->getRepository('AppBundle:Category')->findAll();

        return $this->render('AppBundle:Category:index.html.twig', ['categories'=>$categories]);
    }

    /**
     * @Route("/category/new", name="category_new")
     * @Method({"GET", "POST"})
     */
    public function categoryAction(Request $request)
    {
        $category = new Category();
        $form = $this->createForm(CategoryType::class, $category, [
            'action'=>$this->generateUrl('category_new'),
            'method'=>'POST'
        ]);
        $form->handleRequest($request);
        if ($form->isSubmitted() && $form->isValid()) {            
            $em = $this->getDoctrine()->getManager();
            $em->persist($category);
            $em->flush();

            return $this->redirectToRoute('category_index');
        }

        return $this->render('AppBundle:Category:new.html.twig', ['form'=>$form->createView()]);
    }

    /**
     * @Route("/filter", name="filter_index")
     */
    public function filterIndexAction()
    {
        $filters = $this->getDoctrine()->getRepository('AppBundle:Filter')->findAll();

        return $this->render('AppBundle:Filter:index.html.twig', ['filters'=>$filters]);
    }

    /**
     * @Route("/filter/new", name="filter_new")
     * @Method({"GET", "POST"})
     */
    public function filterAction(Request $request)
    {
        $filter = new Filter();
        $form = $this->createForm(FilterType::class, $filter, [
            'action'=>$this->generateUrl('filter_new'),
            'method'=>'POST'
        ]);
        $form->handleRequest($request);
        if ($form->isSubmitted() && $form->isValid()) {
            $em = $this->getDoctrine()->getManager();
            $em->persist($filter);
            $em->flush();

            return $this->redirectToRoute('filter_index');
        }

        return $this->render('AppBundle:Filter:new.html.twig', ['form'=>$form->createView()]);
    }
}
?>

<!--app/Resources/views/default/index.html.twig ==============================================================================-->
{% extends 'base.html.twig' %}

{% block body %}
    <a href="{{ path('category_index') }}">Category</a> <br>
    <a href="{{ path('filter_index') }}">Filter</a>
{% endblock %}

<!--AppBundle/Resources/views/Category/index.html.twig =======================================================================-->
{% extends '::base.html.twig' %}

{% block body %}
    <ul>
        {% for category in categories %}
            <li>{{ category.name }}</li>
        {% endfor %}
    </ul>

    <a href="{{ path('category_new') }}">New Category</a>
{% endblock %}

<!--AppBundle/Resources/views/Category/new.html.twig =========================================================================-->
{% extends '::base.html.twig' %}

{% block body %}
    {{ form_start(form) }}
        <div>{{ form_row(form.name) }}</div>
        <div>{{ form_row(form.filters) }}</div>
        <button type="submit">Save</button>
    {{ form_end(form) }}
{% endblock %}

<!--AppBundle/Resources/views/Filter/index.html.twig =========================================================================-->
{% extends '::base.html.twig' %}

{% block body %}
    <ul>
        {% for filter in filters %}
            <li>{{ filter.value }}</li>
        {% endfor %}
    </ul>

    <a href="{{ path('filter_new') }}">New Filter</a>
{% endblock %}

<!-- AppBundle/Resources/views/Filter/new.html.twig ==========================================================================-->
{% extends '::base.html.twig' %}

{% block body %}
    {{ form_start(form) }}
        <div>{{ form_row(form.value) }}</div>
        <button type="submit">Save</button>
    {{ form_end(form) }}
{% endblock %}

<!--
When I insert a new filter, all things goes all right, but when I try to insert a new category, I get this error:
Error 500: Could not determine access type for property "filters".

Panic Announcement: Help please! :)
-->

Hello !

Did you find what the problem was ?

Looks related to this : https://knpuniversity.com/screencast/doctrine-relations/create-genus-note ( in the comments section )