Doctrine Patterns (Symfony)

Patterns for working with Doctrine ORM that catch the common pitfalls (N+1, DQL injection, missing indexes, broken migrations). In Doctrine the entity mapping is the source of truth and migrations are generated from it.

1. Entity mapping with attributes

namespace App\Entity;

use App\Repository\SubscriptionRepository;
use Doctrine\DBAL\Types\Types;
use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity(repositoryClass: SubscriptionRepository::class)]
#[ORM\Table(name: 'subscriptions')]
#[ORM\Index(columns: ['user_id', 'status'])]
class Subscription
{
    #[ORM\Id, ORM\GeneratedValue, ORM\Column]
    private ?int $id = null;

    #[ORM\ManyToOne(inversedBy: 'subscriptions')]
    #[ORM\JoinColumn(nullable: false, onDelete: 'CASCADE')]
    private User $user;

    #[ORM\Column(length: 255, unique: true)]
    private string $stripeCustomerId;

    #[ORM\Column(enumType: Status::class)]
    private Status $status;

    #[ORM\Column(type: Types::DECIMAL, precision: 10, scale: 2)]
    private string $amount;

    #[ORM\Column(type: Types::DATETIME_IMMUTABLE)]
    private \DateTimeImmutable $startsAt;
}

• Use enumType: to map a backed enum directly.
• Money/decimals map to Types::DECIMAL (string in PHP — never float).
• Prefer DATETIME_IMMUTABLE over mutable DateTime.
• Declare indexes/unique constraints on the entity — they become migration SQL.

2. Repositories — extend ServiceEntityRepository

namespace App\Repository;

use App\Entity\Subscription;
use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
use Doctrine\Persistence\ManagerRegistry;

/** @extends ServiceEntityRepository<Subscription> */
final class SubscriptionRepository extends ServiceEntityRepository
{
    public function __construct(ManagerRegistry $registry)
    {
        parent::__construct($registry, Subscription::class);
    }

    /** @return Subscription[] */
    public function activeForUser(User $user): array
    {
        return $this->createQueryBuilder('s')
            ->where('s.user = :user')
            ->andWhere('s.status = :status')
            ->setParameter('user', $user)
            ->setParameter('status', Status::Active)
            ->getQuery()
            ->getResult();
    }
}

Keep query logic in repositories — never build queries inline in controllers. Repositories are autowired by type.

3. DQL / QueryBuilder — always parameterized

// Right — named parameters
$em->createQuery('SELECT s FROM App\Entity\Subscription s WHERE s.status = :status')
   ->setParameter('status', Status::Active)
   ->getResult();

// Wrong — string concatenation (DQL injection)
$em->createQuery("SELECT s FROM App\Entity\Subscription s WHERE s.user = " . $userId);  // ❌
$qb->where("s.name = '" . $input . "'");                                                 // ❌

Positional (?1) or named (:name) parameters only. The same rule applies to native SQL via Connection::executeQuery($sql, $params).

4. N+1 prevention — fetch joins

The single most common Doctrine performance bug.

// Problem: lazy relation accessed in a loop → 1 query per row
$subscriptions = $repo->findAll();
foreach ($subscriptions as $s) {
    echo $s->getUser()->getEmail();   // N+1
}

// Solution: JOIN FETCH
$repo->createQueryBuilder('s')
    ->addSelect('u')
    ->join('s.user', 'u')
    ->getQuery()
    ->getResult();

For read-heavy paths, also consider:

• setFetchMode / #[ORM\... fetch: 'EAGER'] only for relations always needed (use sparingly).
• Pagination with Doctrine\ORM\Tools\Pagination\Paginator when fetch-joining to-many relations with LIMIT.

5. Relations — cascade, fetch, orphanRemoval

#[ORM\OneToMany(mappedBy: 'subscription', targetEntity: Invoice::class, cascade: ['persist'], orphanRemoval: true)]
private Collection $invoices;

#[ORM\ManyToOne(fetch: 'LAZY')]
#[ORM\JoinColumn(onDelete: 'CASCADE')]
private User $user;

• Default fetch: 'LAZY' — avoid EAGER (it loads on every query, even when unused).
• cascade: ['persist'] to save children with the parent; avoid cascade: ['remove'] for large graphs — prefer DB onDelete.
• orphanRemoval: true deletes children removed from the collection.
• onDelete on JoinColumn enforces referential action at the DB level (faster than ORM cascade).

6. Identity map and flush discipline

• The EntityManager tracks loaded entities (identity map) — fetching the same row twice returns the same object.
• Call flush() once per unit of work, not per entity. Doctrine batches the SQL inside a transaction.

foreach ($rows as $row) {
    $em->persist($this->toEntity($row));
}
$em->flush();   // single transaction

7. Batch processing — clear to avoid memory blowup

$batchSize = 100;
$q = $em->createQuery('SELECT s FROM App\Entity\Subscription s');
foreach ($q->toIterable() as $i => $subscription) {
    $subscription->expire();
    if (($i % $batchSize) === 0) {
        $em->flush();
        $em->clear();   // detach processed entities, free memory
    }
}
$em->flush();
$em->clear();

8. Lifecycle events / listeners

For cross-cutting concerns (timestamps, audit), prefer entity listeners or event subscribers over fat services.

#[ORM\Entity]
#[ORM\HasLifecycleCallbacks]
class Subscription
{
    #[ORM\PrePersist]
    public function onCreate(): void
    {
        $this->createdAt = new \DateTimeImmutable();
    }
}

For reusable behaviour across entities (e.g. timestampable), use a dedicated #[AsEntityListener] or the StofDoctrineExtensionsBundle rather than copying callbacks.

9. Migrations — generated, then reviewed

• The mapping is the source of truth. Generate: php bin/console doctrine:migrations:diff.
• Review the generated SQL — diff is not always perfect (enum CHECK constraints, defaults, index names).
• Apply: php bin/console doctrine:migrations:migrate. Roll back one: ... migrate prev.
• Verify in sync: php bin/console doctrine:schema:validate.
• Never doctrine:schema:update --force outside throwaway prototypes — it bypasses migration history.

10. Anti-patterns to avoid

Don't	Do
Concatenate input into DQL	setParameter() with :named / ?1
fetch: 'EAGER' everywhere	Lazy by default; JOIN FETCH where needed
Access lazy relations in a loop	Fetch join upstream
float for money	Types::DECIMAL (string in PHP)
flush() inside a loop	flush() once per unit of work
schema:update --force	Generated migrations + review
Query building in controllers	Repository methods

11. Checklist before merging Doctrine changes

• No N+1: loops over relations have a fetch join upstream
• All DQL/QueryBuilder uses bound parameters (no concatenation)
• Indexes declared on FKs and frequently-filtered columns
• Money/decimals use Types::DECIMAL; datetimes are immutable
• Relations have correct fetch (lazy) and onDelete
• flush() called once per unit of work; batch loops clear()
• Migration generated via diff, SQL reviewed, down() reverses cleanly
• doctrine:schema:validate reports in sync