<?php

// src/Entity/User.php

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Security\Core\User\UserInterface;


/**
 * @ORM\Entity(repositoryClass="App\Repository\UserRepository")
 * @ORM\Table(name="user")
 */
class User implements UserInterface, \Serializable
{
    public function serialize()
    {
        return serialize([
            $this->id,
            $this->email,
            $this->password,
        ]);
    }

    public function unserialize($serialized)
    {
        list(
            $this->id,
            $this->email,
            $this->password,
        ) = unserialize($serialized);
    }

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

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

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

    /**
     * @ORM\Column(type="string", length=255)
     */
    private $slug;  // <--- this field will not be unserialized

    // ...

<?php

// src/Entity/Article.php

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity(repositoryClass="App\Repository\ArticleRepository")
 * @ORM\Table(name="article")
 */
class Article
{
    /**
     * @ORM\Id()
     * @ORM\GeneratedValue()
     * @ORM\Column(type="integer")
     */
    private $id;

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

    /**
     * @ORM\ManyToOne(targetEntity="App\Entity\User")
     */
    private $author;  // <-- User object will not be fully unserialized

    // ...

<?php

// src/Controller/EmailController.php

namespace App\Controller;

use Doctrine\ORM\EntityManagerInterface;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Annotation\Route;
use Symfony\Component\Mailer\MailerInterface;
use Symfony\Bridge\Twig\Mime\TemplatedEmail;
use App\Entity\Article;

class EmailController extends AbstractController
{
    /**
     * @Route("/send",
     *     name="send_email")
     *
     * @param Request $request
     * @param EntityManagerInterface $em
     * @param MailerInterface $mailer
     * @return Response
     */
    public function send(Request $request, 
                         EntityManagerInterface $em, 
                         MailerInterface $mailer): Response
    {
        $repository = $em->getRepository(Article::class);
        $article = $repository->find(2);

        // let's imagine that the retrieved Article object with ID = 2 has an associated User with ID = 1 and looks as follows:
        //
        // $author->id = 1;
        // $author->email = "user@example.com";
        // $author->password = "xxxxxx";
        // $author->slug = "user-example-com";
        //
        // $article->id = 2;
        // $article->title = "My article";
        // $article->author = $author; // id = 1

        $context = [];
        $context['article'] = $article;

        $address = $article->getAuthor()->getEmail();  // user@example.com
        $email = (new TemplatedEmail())
            ->from("from@example.com")
            ->to($address)
            ->subject("New article")
            ->htmlTemplate("email/article.html.twig")
            ->context($context);

        $mailer->send($email);

        // at this point, the Article object is serialized and sent to the asynchronous process using Messenger.

        return $this->render('index.html.twig');
    }
}

This is your article:

{{ article.title }}

This is your email:

{{ article.author.email }}

This is your slug:

{{ article.author.slug }}  {# <---- it will render an empty value #}

This is your article:

My article

This is your email:

user@example.com

This is your slug:

// src/Controller/EmailController.php

<?php

namespace App\Controller;

use Doctrine\ORM\EntityManagerInterface;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Annotation\Route;
use Symfony\Component\Mailer\MailerInterface;
use Symfony\Bridge\Twig\Mime\TemplatedEmail;
use Symfony\Bridge\Twig\Mime\BodyRenderer;
use Twig\Environment;
use App\Entity\Article;

class EmailController extends AbstractController
{
    /**
     * @Route("/send",
     *     name="send_email")
     *
     * @param Request $request
     * @param EntityManagerInterface $em
     * @param MailerInterface $mailer
     * @param Environment $twig
     * @return Response
     */
    public function send(Request $request, 
                         EntityManagerInterface $em, 
                         MailerInterface $mailer,
                         Environment $twig): Response
    {
        $repository = $em->getRepository(Article::class);
        $article = $repository->find(2);

        $context = [];
        $context['article'] = $article;

        $address = $article->getAuthor()->getEmail();  // user@example.com
        $email = (new TemplatedEmail())
            ->from("from@example.com")
            ->to($address)
            ->subject("New article")
            ->htmlTemplate("email/article.html.twig")
            ->context($context);

        // ---------------------
        $renderer = new BodyRenderer($twig);
        $renderer->render($email);  // <--- render template before sending to Messenger
        // ---------------------

        $mailer->send($email);

        // this time, the "slug" field of the User object will be correctly filled with "user-example-com".

        return $this->render('index.html.twig');
    }
}

# .env

MAILER_DSN=smtp://user:password@smtp.example.com
MESSENGER_TRANSPORT_DSN=amqp://guest:guest@127.0.0.1:5672/%2f/mailer

# config/packages/mailer.yaml

framework:
    mailer:
        dsn: '%env(MAILER_DSN)%'
        envelope:
            sender: 'sender@example.com'

# config/packages/messenger.yaml

framework:
    messenger:
        transports:
            mailer: '%env(MESSENGER_TRANSPORT_DSN)%'
        routing:
            'Symfony\Component\Mailer\Messenger\SendEmailMessage': mailer