Symfony. Using transactions with 2+ repositories - symfony

Question

In a certain controller I need to do several inserts/updates in the database. I'm using 4 repositories of 4 entities for that, but I need this to be done in a unit of work.
I know that I can do these inserts/updates directly on the controller and use $em->getConnection()->beginTransaction(), $em->getConnection()->rollBack() and $em->getConnection()->commit() to ensure that all or none are done. But this goes against symfony best practices, so I'd like to use repositories.
I've seen this tutorial for Doctrine 1, but it seems like a very complex solution to something that is supposed to be simpler.
Any idea?
Thank you in advance.
edit, I'm using Symfony 3.3.5.
edit2, I add an example of what I want to do. The whole process is to add a message written by a worker to the rest of his department mates, all within a company. The message may or may not have attachments (let's say it does). These files should appear as department and company files, so that even if the department is deleted, the files continue to appear as belonging to the company.
I haven't included error handling in the code to make it simpler.
// New Files
$file1 = new File(); // AppBundle\Entity\File
$file1->setName("name1");
$file2 = new File();
$file2->setName("name2");
$em->getRepository('AppBundle:File')->insert($file1);
$em->getRepository('AppBundle:File')->insert($file2);
// New Post
$post = new Post(); // AppBundle\Entity\Post
$post->setContent($some_text)
->setAuthor($some_user) // AppBundle\Entity\User
->addFile($file1)
->addFile($file2);
$em->getRepository('AppBundle:Post')->insert($post);
// Getting company and department
$company = $em->getRepository('AppBundle:Company')->findOneByCode($some_company_code);
$department = $em->getRepository('AppBundle:Department')->findOneByCode($some_dept_code);
$company->addFile($file1)->addFile($file2);
$department->addFile($file1)->addFile($file2);
$em->getRepository('AppBundle:Company')->update($company);
$em->getRepository('AppBundle:Department')->update($department);
// Notifications
$notif = new Notification(); // AppBundle\Entity\Notification
$notif->setText("Text")
->setPost($post)
->addAddressee($some_dept_member)
->addAddressee($other_dept_member);
$notif = $em->getRepository('AppBundle:Notification')->insert($notif);

Answer 1

In repositories you should only fetch entities or execute queries with DQL that retrieve some data.
Every time you need to create a new row in your db, you have to inform the entity manager that you want to persist the entity, calling $em->persist($myNewEntity). This inform the entity manager that that entity should be persisted to dB.
Once you have created (and persisted) all your entities and/or modified your fetched entities, you can call $em->persist() just once: the entity manager will do all the needed insert/update/delete in a single transaction to your dB.
If you also need to retrieve/fetch data in the same transaction you should consider including all your code in a callback function and pass it to the entity manager, like this
$myCallback=function($em) {
//Do all here: access repositories, create new entities, edit entities
//...
//no need to call $em->flush(), it will be called by Doctrine after the execution of the callback
};
$em->transactional($myCallback) ;