Flashcard step 4 validation

App\Entity\Flashcard.php

<?php
// ...
class Flashcard
{
    /**
     * ...
     * @Assert\NotBlank()
     * @Assert\Length(max="255")
     */
    public $question;

    /**
     * ...
     * @Assert\NotBlank()
     * @Assert\Length(max="255")
     */
    public $answer;

    /**
     * ...
     * @Assert\NotBlank()
     */
    public $lesson;

    //...
}

App\Entity\Lesson.php

<?php

// ...
class Lesson
{
    /**
     * ...
     * @Assert\NotBlank()
     * @Assert\Length(max="255")
     */
    public $name;

    /**
     * ...
     * @Assert\NotBlank()
     */
    public $subject;

   // ...
}

App\Entity\Subject.php

<?php

// ...

class Subject
{
    /**
     * ...
     * @Assert\NotBlank()
     * @Assert\Length(max="255")
     */
    public $name;

    /**
     * ...
     * @Assert\NotBlank(groups={"subjectCreate"})
     */
    public $user;
}

App\Entity\User.php

<?php

// ...

class User implements UserInterface
{

    /**
     * ...
     * @Assert\NotBlank()
     * @Assert\Length(max="255")
     */
    public $name;

    /**
     * ...
     * @Assert\NotBlank()
     * @Assert\Length(max="255")
     */
    public $email;

    /**
     * ...
     * @Assert\NotBlank()
     * @Assert\Length(max="255")
     */
    public $plainPassword;

    // ...
}