Добавить объект в коллекцию типа Symfony 4

Я пытаюсь добавить пользователей в свою форму назначения в Symfony 4, но при попытке загрузить форму получаю следующее исключение:

Could not load type "App\Entity\User": class does not implement "Symfony\Component\Form\FormTypeInterface".

Я написал следующий код:

AssigmentFormType.php:

class AssignmentFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('name')
            ->add('description', TextareaType::class
            )
            ->add('deadline', DateType::class, array(
                'empty_data' => array(
                    'year' => date('Y'), 'month' => date('M'), 'day' => date('d'),
                )
            ))
            ->add('status')
            ->add('story', TextareaType::class)
            ->add('company', EntityType::class, array(
                'class' => Company::class,
                'choice_label' => 'name',
                'placeholder' => 'Select a value',
                'empty_data' => null,
            ))
            ->add('user', CollectionType::class, array(
                'entry_type' => User::class,
                'entry_options' => array('label' => false),
                'allow_add' => true
            ))
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => Assignment::class,
        ]);
    }
}

AssignmentController:

class AssignmentController extends Controller
{
    /**
     * @Security("is_authenticated()")
     * @Route("/assignments", name = "assignments")
     */
    public function index(Request $request)
    {

        $assignment_rep = $this->getDoctrine()->getRepository(Assignment::class);

        if ($request->get('search') == null) {
            $companies = $assignment_rep->findAll();
        }
        else{
            $companies = $assignment_rep->search($request->get('search'));
        }

        $page = $request->query->get('page', 1);

        $adapter = new ArrayAdapter($companies);

        $pagerfanta = new Pagerfanta($adapter);
        $pagerfanta->setMaxPerPage(25);
        $pagerfanta->setCurrentPage($page);

        $companies = $pagerfanta->getCurrentPageResults();

        return $this->render('assignment/index.html.twig', array(
            'assignments' => $companies,
            'pager' => $pagerfanta
        ));
    }

    /**
     * @Route("/assignment/new", name = "newAssignment")
     * @IsGranted("ROLE_ADMIN")
     */
    public function add(Request $request)
    {
        $form = $this->createForm(AssignmentFormType::class);

        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            $this->addFlash('success', 'Assignment added');

            $company = $form->getData();

            $em = $this->getDoctrine()->getManager();
            $em->persist($company);
            $em->flush();

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

        return $this->render('assignment/edit.html.twig', array(
            'form' => $form->createView()
        ));
    }
}

Assignment.php

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

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

    /**
     * @ORM\Column(type = "datetime", nullable=true)
     */
    private $deadline;

    /**
     * @ORM\Column(type = "string", length=255, nullable=true)
     */
    private $description;

    /**
     * @ORM\Column(type = "string", length=255, nullable=true)
     */
    private $status;

    /**
     * @ORM\Column(type = "string", length=255, nullable=true)
     */
    private $story;

    /**
     * @ORM\ManyToOne(targetEntity = "App\Entity\Company", inversedBy = "assignments")
     * @ORM\JoinColumn(nullable=false)
     */
    private $company;

    /**
     * @ORM\ManyToMany(targetEntity = "App\Entity\User", inversedBy = "assignments")
     */
    private $user;

    /**
     * @ORM\OneToMany(targetEntity = "App\Entity\Task", mappedBy = "assignment")
     */
    private $tasks;

    public function __construct()
    {
        $this->user = new ArrayCollection();
        $this->tasks = new ArrayCollection();

    }

    public function getId()
    {
        return $this->id;
    }

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

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

        return $this;
    }

    public function getDeadline(): ?\DateTimeInterface
    {
        return $this->deadline;
    }

    public function setDeadline(?\DateTimeInterface $deadline): self
    {
        $this->deadline = $deadline;

        return $this;
    }

    public function getDescription(): ?string
    {
        return $this->description;
    }

    public function setDescription(?string $description): self
    {
        $this->description = $description;

        return $this;
    }

    public function getStatus(): ?string
    {
        return $this->status;
    }

    public function setStatus(?string $status): self
    {
        $this->status = $status;

        return $this;
    }

    public function getStory(): ?string
    {
        return $this->story;
    }

    public function setStory(?string $story): self
    {
        $this->story = $story;

        return $this;
    }

    public function getCompany(): ?Company
    {
        return $this->company;
    }

    public function setCompany(?Company $company): self
    {
        $this->company = $company;

        return $this;
    }

    /**
     * @return Collection|User[]
     */
    public function getUser(): Collection
    {
        return $this->user;
    }

    public function addUser(User $user): self
    {
        if (!$this->user->contains($user)) {
            $this->user[] = $user;
        }

        return $this;
    }

    public function removeUser(User $user): self
    {
        if ($this->user->contains($user)) {
            $this->user->removeElement($user);
        }

        return $this;
    }

    /**
     * @return Collection|Task[]
     */
    public function getTasks(): Collection
    {
        return $this->tasks;
    }

    public function addTask(Task $task): self
    {
        if (!$this->tasks->contains($task)) {
            $this->tasks[] = $task;
            $task->setAssignment($this);
        }

        return $this;
    }

    public function removeTask(Task $task): self
    {
        if ($this->tasks->contains($task)) {
            $this->tasks->removeElement($task);
            // set the owning side to null (unless already changed)
            if ($task->getAssignment() === $this) {
                $task->setAssignment(null);
            }
        }

        return $this;
    }
}

User.php

class User implements AdvancedUserInterface, \Serializable
{
    /**
     * @ORM\Column(type = "integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy = "AUTO")
     */
    public $id;

    /**
     * @ORM\Column(type = "string", length=25, unique=true)
     */
    public $username;

    /**
     * @ORM\Column(type = "string", length=64)
     */
    public $password;

    /**
     * @ORM\Column(type = "string", length=128, unique=true)
     * @Assert\Email()
     */
    public $email;

    /**
     * @ORM\Column(name = "is_active", type = "boolean")
     */
    public $isActive;

    /**
     * @ORM\Column(name = "roles", type = "array")
     */
    public $roles = [];

    /**
     * @ORM\OneToOne(targetEntity = "App\Entity\UserDetails", mappedBy = "user", cascade = {"persist", "remove"})
     */
    private $userDetails;

    /**
     * @ORM\ManyToMany(targetEntity = "App\Entity\Assignment", mappedBy = "user")
     */
    private $assignments;

    public function __construct()
    {
        $this->isActive = true;
        $this->assignments = new ArrayCollection();
        // may not be needed, see section on salt below
        // $this->salt = md5(uniqid('', true));
    }

    /**
     * @param mixed $isActive
     */
    public function setIsActive($isActive)
    {
        $this->isActive = $isActive;
    }


    public function getEmail()
    {
        return $this->email;
    }

    public function setEmail($email)
    {
        $this->email = $email;
    }

    public function getUsername()
    {
        return $this->username;
    }

    public function setUsername($username)
    {
        $this->username = $username;
    }

    public function getPassword()
    {
        return $this->password;
    }

    public function setPassword($password)
    {
        $this->password = $password;
    }

    public function getSalt()
    {
        // The bcrypt and argon2i algorithms don't require a separate salt.
        // You *may* need a real salt if you choose a different encoder.
        return null;
    }


    public function getRoles()
    {
        $roles = $this->roles;

        if (!in_array('ROLE_USER', $roles)) {
            $roles[] = 'ROLE_USER';
        }
        return $roles;
    }

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

    public function setRoles(array $roles)
    {
        $this->roles = $roles;
    }

    public function eraseCredentials()
    {
    }

    /** @see \Serializable::serialize() */
    public function serialize()
    {
        return serialize(array(
            $this->id,
            $this->username,
            $this->password,
            $this->isActive
            // see section on salt below
            // $this->salt,
        ));
    }

    /** @see \Serializable::unserialize() */
    public function unserialize($serialized)
    {
        list (
            $this->id,
            $this->username,
            $this->password,
            $this->isActive
            // see section on salt below
            // $this->salt
            ) = unserialize($serialized, ['allowed_classes' => false]);
    }

    public function isAccountNonExpired()
    {
        return true;
    }

    public function isAccountNonLocked()
    {
        return true;
    }

    public function isCredentialsNonExpired()
    {
        return true;
    }

    public function isEnabled()
    {
        return $this->isActive;
    }

    public function getUserDetails(): ?UserDetails
    {
        return $this->userDetails;
    }

    public function setUserDetails(UserDetails $userDetails): self
    {
        $this->userDetails = $userDetails;

        // set the owning side of the relation if necessary
        if ($this !== $userDetails->getUser()) {
            $userDetails->setUser($this);
        }

        return $this;
    }

    /**
     * @return Collection|Assignment[]
     */
    public function getAssignments(): Collection
    {
        return $this->assignments;
    }

    public function addAssignment(Assignment $assignment): self
    {
        if (!$this->assignments->contains($assignment)) {
            $this->assignments[] = $assignment;
            $assignment->addUser($this);
        }

        return $this;
    }

    public function removeAssignment(Assignment $assignment): self
    {
        if ($this->assignments->contains($assignment)) {
            $this->assignments->removeElement($assignment);
            $assignment->removeUser($this);
        }

        return $this;
    }
}

Проблема решается удалением атрибута allow_add => true в массиве, но таким образом я не могу добавлять пользователей в свое назначение. Может ли кто-нибудь помочь мне добавить пользователей в ассигмнет в форме?