Symfony Domain Modeling

Symfony Domain Modeling

You are an expert in Domain-Driven Design within Symfony hexagonal architecture. Use this skill when users need to create or modify domain model elements.

When to Activate

• User wants to create an entity or aggregate root
• User asks about value objects
• User needs domain events
• User mentions domain exceptions or business rules
• User discusses aggregates or invariants

Entity Patterns

Rules

• Private constructor + static factory method(s)
• No Doctrine annotations/attributes — mapping is in Infrastructure
• Record domain events on state changes
• Protect invariants in methods, not in external validators
• Use value objects for typed properties (no primitive obsession)

Template

namespace App\Domain\{Module}\Entity;

final class {Entity}
{
    private array $domainEvents = [];

    private function __construct(
        private {EntityId} $id,
        // typed properties using value objects
    ) {
    }

    public static function create(/* params */): self
    {
        // validate invariants
        $entity = new self(/* params */);
        $entity->recordEvent(new {Entity}Created(/* event data */));
        return $entity;
    }

    // Business methods that protect invariants
    public function doAction(/* params */): void
    {
        // guard clause / invariant check
        // state change
        $this->recordEvent(new {ActionDone}(/* event data */));
    }

    public function pullDomainEvents(): array
    {
        $events = $this->domainEvents;
        $this->domainEvents = [];
        return $events;
    }

    private function recordEvent(object $event): void
    {
        $this->domainEvents[] = $event;
    }
}

Value Object Patterns

Rules

• Always final readonly class
• Self-validating in constructor (throw domain exception if invalid)
• Immutable — no setters
• Implement equals() for comparison
• Use for: IDs, email, money, dates, status enums

Template

namespace App\Domain\{Module}\ValueObject;

use App\Domain\{Module}\Exception\Invalid{ValueObject}Exception;

final readonly class {ValueObject}
{
    public function __construct(
        public string $value,
    ) {
        if (/* validation fails */) {
            throw new Invalid{ValueObject}Exception($value);
        }
    }

    public function equals(self $other): bool
    {
        return $this->value === $other->value;
    }

    public function __toString(): string
    {
        return $this->value;
    }
}

Domain Event Patterns

Rules

• Past-tense naming: UserRegistered, OrderPlaced, PaymentFailed
• Immutable (final readonly class)
• Carry only the data needed by handlers (IDs, not full entities)
• Created inside entities on state changes

Template

namespace App\Domain\{Module}\Event;

final readonly class {PastTenseEvent}
{
    public function __construct(
        public string $entityId,
        public \DateTimeImmutable $occurredAt = new \DateTimeImmutable(),
        // additional event-specific data
    ) {
    }
}

Domain Exception Patterns

namespace App\Domain\{Module}\Exception;

final class {DomainException} extends \DomainException
{
    public static function because{Reason}(/* context */): self
    {
        return new self(sprintf('Descriptive message: %s', $context));
    }
}

References

See references/ for detailed patterns:

• entity-patterns.md — Full entity examples with aggregates
• value-object-patterns.md — Common value object implementations
• domain-event-patterns.md — Event design and dispatch patterns