In Symfony, the entity manager is responsible for managing the persistence of entities in a database. It allows you to create, update, delete, and retrieve entities from the database. However, despite its convenience, using the entity manager in your application can be a bad idea. In this article, we'll discuss why it's generally considered a better practice to use explicit entity repositories instead of the entity manager.

First, let's define what we mean by "explicit entity repositories". In Symfony, an entity repository is a class that is specifically designed to handle the persistence of a specific entity type. It acts as a layer between your application and the database, allowing you to write custom queries and operations that are specific to your entity.

On the other hand, the entity manager is a generic tool that can be used to persist any entity. It's essentially a catch-all solution for handling database operations, and as such, it can become a bottleneck in your application. 

A typical usage of the Entity Manager in Symfony would look like this:

php
use Doctrine\ORM\EntityManagerInterface;

class SomeClass
{
    private $entityManager;

    public function __construct(EntityManagerInterface $entityManager)
    {
        $this->entityManager = $entityManager;
    }

    public function updateUserName($userId, $newName)
    {
        // Find a user by their ID
        $user = $this->entityManager->find(User::class, $userId);

        // Update the user's name
        $user->setName($newName);

        // Persist the changes to the database
        $this->entityManager->persist($user);
        $this->entityManager->flush();
    }
}

In this example, we would create the Repository Class individually. This is not just exceptionally bad design, but it will also make changes to the given repository much harder to implement, as we cannot use resolving of the Entities via our IDE.

One of the main reasons why using the entity manager is a bad idea is that it can make your code harder to understand and maintain. When you use the entity manager, your code becomes tightly coupled to the persistence layer of your application. This can make it harder to understand what's happening in your code, and it can also make it more difficult to change the way you persist your entities in the future.

On the other hand, when you use explicit entity repositories, your code becomes decoupled from the persistence layer. This makes it easier to understand what's happening in your code, and it also makes it easier to change the way you persist your entities in the future.

Another reason why using the entity manager is a bad idea is that it can make your application slower. The entity manager is a generic tool that can be used to persist any entity, and as such, it has to handle a lot of different cases. This can lead to slower performance, especially if you have a large number of entities or if you're performing a lot of database operations.

On the other hand, explicit entity repositories are designed to handle the persistence of a specific entity type, and as such, they can be optimized for the specific needs of that entity. This can lead to faster performance, especially if you have a large number of entities or if you're performing a lot of database operations.

Certainly! Here is the revised code example that uses an explicit entity repository and does not use the entity manager:

php
use App\Repository\UserRepository;

class SomeClass
{
    private $userRepository;

    public function __construct(UserRepository $userRepository)
    {
        $this->userRepository = $userRepository;
    }

    public function updateUserName($userId, $newName)
    {
        // Find a user by their ID
        $user = $this->userRepository->find($userId);

        // Update the user's name
        $user->setName($newName);

        // Persist the changes to the database
        $this->userRepository->persist($user);
        $this->userRepository->flush();
    }
}

In this revised example, the `SomeClass` class no longer relies on the entity manager. Instead, it uses the `UserRepository` to find the user, update its name, and save the changes to the database. This allows you to use an explicit entity repository to handle all of the persistence logic for the User entity, without the need to use the entity manager. In Symfony 6 the Repository classes all have access to the relevant persistence functions, thus we don't really need to utilize the EntityManager at all. Instead, we can completly rely on our explicit entity repositories to do the job for us. Also, it will become clear for other developers what kind of transaction is going on here by looking at the property name instead of scanning the code for EntityManager declarations.

One final reason why using the entity manager is a bad idea is that it can make your code harder to test. When you use the entity manager, your code becomes tightly coupled to the persistence layer of your application, which can make it harder to mock or stub out the entity manager in your tests. On the other hand, when you use explicit entity repositories, your code becomes decoupled from the persistence layer, which makes it easier to mock or stub out the entity repository in your tests.

In conclusion, while the entity manager can be a convenient tool for handling the persistence of entities in a Symfony application, it's generally considered a bad idea to use it. Instead, it's generally a better practice to use explicit entity repositories, which can make your code easier to understand, maintain, and test, and can also lead to faster performance.