App\Entity\Flashcard.php

<?php
// ..
use Symfony\Component\Serializer\Annotation\Groups;

/**
 * @ApiResource(
 *     normalizationContext={"groups"={"flashcardList"}},
 *     denormalizationContext={"groups"={"flashcardCreate"}}
 * )
 * ...
 */
class Flashcard
{
    /**
     * ...
     * @Groups({"flashcardList"})
     */
    private $id;

    /**
     * ...
     * @Groups({"flashcardList", "flashcardCreate"})
     */
    public $question;

    /**
     * ...
     * @Groups({"flashcardList", "flashcardCreate"})
     */
    public $answer;

    /**
     * ...
     * @Groups({"flashcardCreate"})
     */
    public $lesson;

    // ...
}