Как предварительно установить сгенерированные флажки в Symfony 4?

Если поля вашей формы сопоставлены с сущностью, данные должны быть установлены автоматически. Вам даже не нужно вручную добавлять категории в сообщение.

Вот полный образец рабочего кода, который соответствует тому, что вы хотите сделать:

//src/Entity/Group.php
namespace App\Entity;

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

/**
 * @ORM\Entity(repositoryClass = "App\Repository\GroupRepository")
 * @ORM\Table(name = "`group`")
 */
class Group
{
    /**
     * Group constructor.
     */
    public function __construct()
    {
        $this->players = new ArrayCollection();
    }

    /**
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type = "integer")
     */
    private $id;

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

    /**
     * @ORM\ManyToMany(targetEntity = "App\Entity\Player", inversedBy = "groups")
     */
    private $players;

    /**
     * @return mixed
     */
    public function getPlayers()
    {
        return $this->players;
    }

    /**
     * @param mixed $players
     *
     * @return Group
     */
    public function setPlayers($players)
    {
        $this->players = $players;

        return $this;
    }

    /**
     * @param Player $player
     *
     * @return Group
     */
    public function addPlayer(Player $player)
    {
        $this->players->add($player);

        return $this;
    }

    /**
     * @param Player $player
     *
     * @return Group
     */
    public function removePlayer(Player $player)
    {
        $this->players->removeElement($player);

        return $this;
    }
}

//src/Entity/Player.php
namespace App\Entity;

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

/**
 * @ORM\Entity(repositoryClass = "App\Repository\PlayerRepository")
 */
class Player
{
    /**
     * @return string
     */
    public function __toString()
    {
        return $this->name;
    }

    /**
     * Player constructor.
     */
    public function __construct()
    {
        $this->groups = new ArrayCollection();
    }

    /**
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type = "integer")
     */
    private $id;

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

    /**
     * @ORM\ManyToMany(targetEntity = "App\Entity\Group", mappedBy = "players")
     */
    private $groups;

    /**
     * @return mixed
     */
    public function getGroups()
    {
        return $this->groups;
    }

    /**
     * @param mixed $groups
     *
     * @return Player
     */
    public function setGroups($groups)
    {
        $this->groups = $groups;

        return $this;
    }

    /**
     * @param Group $group
     *
     * @return Player
     */
    public function addGroup(Group $group)
    {
        $this->groups->add($group);

        return $this;
    }

    /**
     * @param Group $group
     *
     * @return Player
     */
    public function removeGroup(Group $group)
    {
        $this->groups->removeElement($group);

        return $this;
    }
}

//src/Form/GroupType.php
namespace App\Form;

use App\Entity\Group;
use App\Entity\Player;
use Symfony\Bridge\Doctrine\Form\Type\EntityType;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class GroupType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add(
                'players',
                EntityType::class,
                [
                    'class' => Player::class,
                    'multiple' => true,
                    'expanded' => true,
                    'required' => false,
                ]
            );
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(
            [
                'data_class' => Group::class,
            ]
        );
    }
}

//src/Controller/GroupController.php
namespace App\Controller;

use App\Entity\Group;
use App\Form\GroupType;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Annotation\Route;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;

class GroupController extends Controller
{
    /**
     * @Route("/edit-group/{id}", name = "group_edit")
     * @param Request $request
     * @param Group $group
     *
     * @return Response
     */
    public function editGroup(Request $request, Group $group)
    {
        $em = $this->getDoctrine()->getManager();
        $form = $this->createForm(GroupType::class, $group);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {

            $em->persist($group);
            $em->flush();
        }

        return new Response(
            $this->renderView(
                'group/edit-group.html.twig',
                array(
                    'form' => $form->createView(),
                    )
            )
        );
    }
}

//templates/group/edit-group.html.twig
{% extends 'base.html.twig' %}

{% block body %}
<div class = "container">
    <div class = "row">
        <h1>Edit group</h1>
    </div>

    {{ form_start(form) }}

    {{ form_row(form.players) }}

    <button type = "submit" class = "btn btn-success">Edit</button>

    {{ form_rest(form) }}
    {{ form_end(form) }}
</div>
{% endblock %}