Stop Using Entities for Validation: Symfony’s DTO-First Approach

We often associate the Validator component with one primary task: validating entities bound to forms. It’s the trusty gatekeeper that ensures NotBlank, Email, and Length constraints are met before our data hits the database. Validator NotBlank Email Length While this is its bread and butter, the Validator is a far more powerful and versatile tool. Its capabilities extend well beyond the Form component, enabling us to build robust, decoupled, and highly maintainable validation logic for any data structure imaginable. This article dives deep into the less-traveled paths of the Symfony Validator. We’ll move past basic entity checks and explore advanced patterns that solve complex, real-world problems. We’ll be building sophisticated validation for API endpoints using DTOs, implementing dynamic validation flows with Group Sequences, wrangling complex nested data structures, and creating custom constraints with service dependencies. Symfony Validator This guide is for developers who are comfortable with Symfony and want to elevate their validation skills. We will be working with Symfony and modern PHP version, leveraging modern features like PHP attributes for a clean and declarative approach. Setup and Prerequisites Before we begin, let’s ensure our project is set up correctly. These examples assume you have a working Symfony application. The core component we need is symfony/validator. You can install it via Composer: composer require symfony/validator composer require symfony/validator In a standard Symfony Flex application, this component is likely already installed as part of symfony/framework-bundle. Your composer.json should contain something similar to this: composer.json { "require": { "php": ">=8.2", "symfony/framework-bundle": "7.3.*", "symfony/validator": "7.3.*", /* ... other dependencies */ } } { "require": { "php": ">=8.2", "symfony/framework-bundle": "7.3.*", "symfony/validator": "7.3.*", /* ... other dependencies */ } } The validator is typically enabled by default. Your configuration in config/packages/validator.yaml should look like this, enabling attribute mapping: config/packages/validator.yaml # config/packages/validator.yaml framework: validation: enabled: true # Enables validator discovery of constraints configured via PHP attributes. enable_attributes: true # config/packages/validator.yaml framework: validation: enabled: true # Enables validator discovery of constraints configured via PHP attributes. enable_attributes: true With our setup confirmed, let’s dive in. Bulletproof API Endpoints with DTO Validation A common anti-pattern in API development is to directly use Doctrine entities as the input and output for your controllers. This creates tight coupling between your API contract and your database schema, and it can open the door to mass-assignment vulnerabilities. A much cleaner approach is to use Data Transfer Objects (DTOs). A DTO is a simple class that represents the data structure for a specific operation, like user registration or updating a product. It’s a “middleman” for your data. Data Transfer Objects (DTOs) “middleman” By applying validation constraints to a DTO instead of an entity, we gain several advantages: Decoupling: Your API contract (the DTO) is independent of your persistence layer (the Entity). Clarity: The DTO explicitly defines the expected input for an endpoint. Security: You only map the fields from the DTO to the entity that are meant to be changed, preventing unwanted updates. Decoupling: Your API contract (the DTO) is independent of your persistence layer (the Entity). Decoupling: Clarity: The DTO explicitly defines the expected input for an endpoint. Clarity: Security: You only map the fields from the DTO to the entity that are meant to be changed, preventing unwanted updates. Security: Let’s create a DTO for a new user registration endpoint. This class is a simple container for the incoming data, decorated with validation attributes. // src/Dto/UserRegistrationDto.php namespace App\Dto; use Symfony\Component\Validator\Constraints as Assert; class UserRegistrationDto { public function __construct( #[Assert\NotBlank] #[Assert\Email(message: 'The email {{ value }} is not a valid email.')] public readonly ?string $email, #[Assert\NotBlank] #[Assert\Length(min: 8, minMessage: 'Your password must be at least {{ limit }} characters long')] #[Assert\Regex( pattern: '/^(?=.*[A-Z])(?=.*[a-z])(?=.*\d).+$/', message: 'Password must contain at least one uppercase letter, one lowercase letter, and one number.' )] public readonly ?string $password, #[Assert\NotBlank] public readonly ?string $firstName, ) { } } // src/Dto/UserRegistrationDto.php namespace App\Dto; use Symfony\Component\Validator\Constraints as Assert; class UserRegistrationDto { public function __construct( #[Assert\NotBlank] #[Assert\Email(message: 'The email {{ value }} is not a valid email.')] public readonly ?string $email, #[Assert\NotBlank] #[Assert\Length(min: 8, minMessage: 'Your password must be at least {{ limit }} characters long')] #[Assert\Regex( pattern: '/^(?=.*[A-Z])(?=.*[a-z])(?=.*\d).+$/', message: 'Password must contain at least one uppercase letter, one lowercase letter, and one number.' )] public readonly ?string $password, #[Assert\NotBlank] public readonly ?string $firstName, ) { } } In our controller, we’ll deserialize the incoming JSON request into our UserRegistrationDto. We then inject the ValidatorInterface service to manually run the validation. If validation fails, we return a structured 422 Unprocessable Entity response. UserRegistrationDto ValidatorInterface // src/Controller/RegistrationController.php namespace App\Controller; use App\Dto\UserRegistrationDto; use Symfony\Bundle\FrameworkBundle\Controller\AbstractController; use Symfony\Component\HttpFoundation\JsonResponse; use Symfony\Component\HttpFoundation\Request; use Symfony\Component\HttpFoundation\Response; use Symfony\Component\HttpKernel\Attribute\MapRequestPayload; use Symfony\Component\Routing\Annotation\Route; use Symfony\Component\Validator\Validator\ValidatorInterface; class RegistrationController extends AbstractController { #[Route('/api/register', name: 'api_register', methods: ['POST'])] public function register( #[MapRequestPayload] UserRegistrationDto $dto, // Symfony automatically deserializes and validates ValidatorInterface $validator ): JsonResponse { // As of Symfony 6.3, MapRequestPayload automatically validates and throws an exception on failure. // The manual validation block below is still useful for more complex, custom logic // or if you want to control the error response format completely. $violations = $validator->validate($dto); if (count($violations) > 0) { $errors = []; foreach ($violations as $violation) { $errors[$violation->getPropertyPath()][] = $violation->getMessage(); } return new JsonResponse(['errors' => $errors], Response::HTTP_UNPROCESSABLE_ENTITY); } // DTO is valid, proceed with creating the user entity, etc. // $user = new User(); // $user->setEmail($dto->email); // ... return new JsonResponse( ['message' => 'User registered successfully!'], Response::HTTP_CREATED ); } } // src/Controller/RegistrationController.php namespace App\Controller; use App\Dto\UserRegistrationDto; use Symfony\Bundle\FrameworkBundle\Controller\AbstractController; use Symfony\Component\HttpFoundation\JsonResponse; use Symfony\Component\HttpFoundation\Request; use Symfony\Component\HttpFoundation\Response; use Symfony\Component\HttpKernel\Attribute\MapRequestPayload; use Symfony\Component\Routing\Annotation\Route; use Symfony\Component\Validator\Validator\ValidatorInterface; class RegistrationController extends AbstractController { #[Route('/api/register', name: 'api_register', methods: ['POST'])] public function register( #[MapRequestPayload] UserRegistrationDto $dto, // Symfony automatically deserializes and validates ValidatorInterface $validator ): JsonResponse { // As of Symfony 6.3, MapRequestPayload automatically validates and throws an exception on failure. // The manual validation block below is still useful for more complex, custom logic // or if you want to control the error response format completely. $violations = $validator->validate($dto); if (count($violations) > 0) { $errors = []; foreach ($violations as $violation) { $errors[$violation->getPropertyPath()][] = $violation->getMessage(); } return new JsonResponse(['errors' => $errors], Response::HTTP_UNPROCESSABLE_ENTITY); } // DTO is valid, proceed with creating the user entity, etc. // $user = new User(); // $user->setEmail($dto->email); // ... return new JsonResponse( ['message' => 'User registered successfully!'], Response::HTTP_CREATED ); } } Note on MapRequestPayload: Since Symfony 6.3, the #[MapRequestPayload] attribute can automatically deserialize the request body into your DTO and run the validator. If validation fails, it throws a HttpException which results in a 422 response with a detailed violation list, simplifying the controller logic significantly. The manual validation shown above is still incredibly useful when you need to perform additional checks or customize the error response structure beyond what Symfony provides by default. MapRequestPayload #[MapRequestPayload] To test this, send a POST request to /api/register with an invalid payload: /api/register curl -X POST http://localhost:8000/api/register \ -H "Content-Type: application/json" \ -d '{ "email": "not-an-email", "password": "short", "firstName": "" }' curl -X POST http://localhost:8000/api/register \ -H "Content-Type: application/json" \ -d '{ "email": "not-an-email", "password": "short", "firstName": "" }' You will receive a structured JSON error response with a 422 status code: 422 { "errors": { "email": [ "The email \"not-an-email\" is not a valid email." ], "password": [ "Your password must be at least 8 characters long", "Password must contain at least one uppercase letter, one lowercase letter, and one number." ], "firstName": [ "This value should not be blank." ] } } { "errors": { "email": [ "The email \"not-an-email\" is not a valid email." ], "password": [ "Your password must be at least 8 characters long", "Password must contain at least one uppercase letter, one lowercase letter, and one number." ], "firstName": [ "This value should not be blank." ] } } This pattern provides a robust and scalable way to handle API validation, keeping your core domain logic clean and your API contracts explicit. Dynamic Logic with Group Sequences What if your validation rules are not static? Consider a registration form where selecting “Corporate” as the account type makes a “CompanyName” field required. If “Individual” is selected, “CompanyName” should be ignored. Corporate CompanyName required Individual CompanyName ignored This is a classic case of conditional validation. While you could use complex Expression constraints, a more powerful and maintainable solution is a Group Sequence. A Group Sequence allows you to define an ordered list of validation groups. The validator processes them one by one and stops at the first group that contains a violation. We’ll create a UserOnboardingDto where the required fields change based on the accountType. UserOnboardingDto accountType First, we need to define our groups. We can do this using an interface or a simple class with constants. // src/Validator/OnboardingGroups.php namespace App\Validator; interface OnboardingGroups { public const DEFAULT = 'Default'; public const INDIVIDUAL = 'Individual'; public const CORPORATE = 'Corporate'; } // src/Validator/OnboardingGroups.php namespace App\Validator; interface OnboardingGroups { public const DEFAULT = 'Default'; public const INDIVIDUAL = 'Individual'; public const CORPORATE = 'Corporate'; } Next, create a Group Sequence Provider. This class will contain the logic to decide which validation groups should run, and in what order, based on the object’s state. // src/Dto/UserOnboardingDto.php namespace App\Dto; use App\Validator\OnboardingGroups; use Symfony\Component\Validator\Constraints as Assert; use Symfony\Component\Validator\GroupSequenceProviderInterface; #[Assert\GroupSequenceProvider] class UserOnboardingDto implements GroupSequenceProviderInterface { public const TYPE_INDIVIDUAL = 'individual'; public const TYPE_CORPORATE = 'corporate'; #[Assert\NotBlank(groups: [OnboardingGroups::DEFAULT])] #[Assert\Choice( choices: [self::TYPE_INDIVIDUAL, self::TYPE_CORPORATE], groups: [OnboardingGroups::DEFAULT] )] public ?string $accountType = null; #[Assert\NotBlank(groups: [OnboardingGroups::DEFAULT])] public ?string $name = null; #[Assert\NotBlank(groups: [OnboardingGroups::CORPORATE])] #[Assert\Length(min: 2, groups: [OnboardingGroups::CORPORATE])] public ?string $companyName = null; public function getGroupSequence(): array { // Always validate the 'Default' group first. $sequence = [OnboardingGroups::DEFAULT]; // Conditionally add the next group based on the account type. if ($this->accountType === self::TYPE_CORPORATE) { $sequence[] = OnboardingGroups::CORPORATE; } elseif ($this->accountType === self::TYPE_INDIVIDUAL) { $sequence[] = OnboardingGroups::INDIVIDUAL; } return $sequence; } } // src/Dto/UserOnboardingDto.php namespace App\Dto; use App\Validator\OnboardingGroups; use Symfony\Component\Validator\Constraints as Assert; use Symfony\Component\Validator\GroupSequenceProviderInterface; #[Assert\GroupSequenceProvider] class UserOnboardingDto implements GroupSequenceProviderInterface { public const TYPE_INDIVIDUAL = 'individual'; public const TYPE_CORPORATE = 'corporate'; #[Assert\NotBlank(groups: [OnboardingGroups::DEFAULT])] #[Assert\Choice( choices: [self::TYPE_INDIVIDUAL, self::TYPE_CORPORATE], groups: [OnboardingGroups::DEFAULT] )] public ?string $accountType = null; #[Assert\NotBlank(groups: [OnboardingGroups::DEFAULT])] public ?string $name = null; #[Assert\NotBlank(groups: [OnboardingGroups::CORPORATE])] #[Assert\Length(min: 2, groups: [OnboardingGroups::CORPORATE])] public ?string $companyName = null; public function getGroupSequence(): array { // Always validate the 'Default' group first. $sequence = [OnboardingGroups::DEFAULT]; // Conditionally add the next group based on the account type. if ($this->accountType === self::TYPE_CORPORATE) { $sequence[] = OnboardingGroups::CORPORATE; } elseif ($this->accountType === self::TYPE_INDIVIDUAL) { $sequence[] = OnboardingGroups::INDIVIDUAL; } return $sequence; } } The #[Assert\GroupSequenceProvider] attribute tells the validator to call the getGroupSequence() method on this object to determine the validation plan. #[Assert\GroupSequenceProvider] getGroupSequence() Our getGroupSequence() method first adds the Default group. The validator will always run constraints in this group. getGroupSequence() If the accountType is ‘corporate’, it then adds the Corporate group to the sequence. corporate Corporate group Constraints are linked to groups. companyName is only NotBlank if the Corporate group is active. companyName NotBlank Trigger the Validation In your controller or service, you validate the DTO as usual. The validator will automatically detect and use the group sequence provider. // In a controller or service... $dto = new UserOnboardingDto(); $dto->accountType = 'corporate'; $dto->name = 'John Doe'; // $dto->companyName is left null $violations = $validator->validate($dto); // This will produce one violation for the missing companyName. foreach ($violations as $violation) { echo $violation->getPropertyPath() . ': ' . $violation->getMessage() . "\n"; } // Output: companyName: This value should not be blank. // In a controller or service... $dto = new UserOnboardingDto(); $dto->accountType = 'corporate'; $dto->name = 'John Doe'; // $dto->companyName is left null $violations = $validator->validate($dto); // This will produce one violation for the missing companyName. foreach ($violations as $violation) { echo $violation->getPropertyPath() . ': ' . $violation->getMessage() . "\n"; } // Output: companyName: This value should not be blank. If we set $dto->accountType = ‘individual’;, the Corporate group is never added to the sequence, and no violation is triggered for companyName. This powerful pattern lets you build complex, multi-step validation logic directly into your data objects. $dto->accountType = ‘individual’; Corporate companyName Validating Nested Objects and Collections Modern applications often deal with complex data structures, such as an order with a list of line items, or a raw JSON payload with a specific key-value structure. The validator provides excellent tools for this: #[Valid] for validating collections of objects and #[Collection] for validating array maps. #[Valid] #[Collection] **Validating an Array of Objects with #[Valid]**The #[Valid] constraint tells the validator to “dive into” a nested object or a collection of objects and validate each one against its own constraints. #[Valid] Let’s model a PurchaseOrderDto that contains an array of OrderItemDto objects. PurchaseOrderDto OrderItemDto // src/Dto/OrderItemDto.php namespace App\Dto; use Symfony\Component\Validator\Constraints as Assert; class OrderItemDto { #[Assert\NotBlank] #[Assert\Length(min: 3)] public ?string $sku = null; #[Assert\NotBlank] #[Assert\Range(min: 1, max: 100)] public ?int $quantity = null; } // src/Dto/PurchaseOrderDto.php namespace App\Dto; use Symfony\Component\Validator\Constraints as Assert; class PurchaseOrderDto { #[Assert\NotBlank] public ?string $orderId = null; /** * @var OrderItemDto[] */ #[Assert\Count(min: 1, minMessage: 'An order must contain at least one item.')] #[Assert\Valid] // This is the key! public array $items = []; } // src/Dto/OrderItemDto.php namespace App\Dto; use Symfony\Component\Validator\Constraints as Assert; class OrderItemDto { #[Assert\NotBlank] #[Assert\Length(min: 3)] public ?string $sku = null; #[Assert\NotBlank] #[Assert\Range(min: 1, max: 100)] public ?int $quantity = null; } // src/Dto/PurchaseOrderDto.php namespace App\Dto; use Symfony\Component\Validator\Constraints as Assert; class PurchaseOrderDto { #[Assert\NotBlank] public ?string $orderId = null; /** * @var OrderItemDto[] */ #[Assert\Count(min: 1, minMessage: 'An order must contain at least one item.')] #[Assert\Valid] // This is the key! public array $items = []; } When we validate an instance of PurchaseOrderDto, the #[Valid] constraint on the $items property instructs the validator to iterate through the array and validate each OrderItemDto object inside it. PurchaseOrderDto #[Valid] OrderItemDto $orderItem1 = new OrderItemDto(); $orderItem1->sku = 'ABC-123'; $orderItem1->quantity = 2; $orderItem2 = new OrderItemDto(); $orderItem2->sku = 'XY'; // Invalid length $orderItem2->quantity = 200; // Invalid range $order = new PurchaseOrderDto(); $order->orderId = 'PO-2025-10'; $order->items = [$orderItem1, $orderItem2]; $violations = $validator->validate($order); // Violations will be reported with nested property paths foreach ($violations as $violation) { echo $violation->getPropertyPath() . ': ' . $violation->getMessage() . "\n"; } $orderItem1 = new OrderItemDto(); $orderItem1->sku = 'ABC-123'; $orderItem1->quantity = 2; $orderItem2 = new OrderItemDto(); $orderItem2->sku = 'XY'; // Invalid length $orderItem2->quantity = 200; // Invalid range $order = new PurchaseOrderDto(); $order->orderId = 'PO-2025-10'; $order->items = [$orderItem1, $orderItem2]; $violations = $validator->validate($order); // Violations will be reported with nested property paths foreach ($violations as $violation) { echo $violation->getPropertyPath() . ': ' . $violation->getMessage() . "\n"; } This will produce the following output, clearly indicating where the errors are in the nested structure: items[1].sku: This value is too short. It should have 3 characters or more. items[1].quantity: This value should be between 1 and 100. items[1].sku: This value is too short. It should have 3 characters or more. items[1].quantity: This value should be between 1 and 100. **Validating Key-Value Arrays with #[Collection]**Sometimes you’re not dealing with objects, but with associative arrays, like decoded JSON or configuration data. The #[Collection] constraint is perfect for this. It allows you to define a set of constraints for a key-value map. #[Collection] Imagine you’re receiving a settings payload that must have specific keys. // In a service or controller... use Symfony\Component\Validator\Constraints as Assert; $data = [ 'theme' => 'dark', 'notifications' => [ 'email' => 'daily', 'push' => true, ], 'unknown_field' => 'some value' // Extra field ]; $constraint = new Assert\Collection([ 'fields' => [ 'theme' => [ new Assert\NotBlank(), new Assert\Choice(['dark', 'light']), ], 'notifications' => new Assert\Collection([ 'email' => new Assert\Choice(['daily', 'weekly', 'none']), 'push' => new Assert\Type('bool'), ]), 'language' => new Assert\Required([ // This key is required but missing new Assert\NotBlank(), new Assert\Choice(['en', 'fr', 'de']), ]), ], 'allowExtraFields' => false, 'missingFieldsMessage' => 'The field {{ field }} is missing.', ]); $violations = $validator->validate($data, $constraint); // In a service or controller... use Symfony\Component\Validator\Constraints as Assert; $data = [ 'theme' => 'dark', 'notifications' => [ 'email' => 'daily', 'push' => true, ], 'unknown_field' => 'some value' // Extra field ]; $constraint = new Assert\Collection([ 'fields' => [ 'theme' => [ new Assert\NotBlank(), new Assert\Choice(['dark', 'light']), ], 'notifications' => new Assert\Collection([ 'email' => new Assert\Choice(['daily', 'weekly', 'none']), 'push' => new Assert\Type('bool'), ]), 'language' => new Assert\Required([ // This key is required but missing new Assert\NotBlank(), new Assert\Choice(['en', 'fr', 'de']), ]), ], 'allowExtraFields' => false, 'missingFieldsMessage' => 'The field {{ field }} is missing.', ]); $violations = $validator->validate($data, $constraint); fields: Defines the expected keys and the constraints for their values. You can nest Collection constraints for multi-dimensional arrays. Required: A wrapper constraint to ensure a key is present. allowExtraFields: When false, it flags any keys in the data that are not defined in the fields option. allowMissingFields: When false (default), it flags any keys defined in fields that are missing from the data. Using Required provides more granular control. fields: Defines the expected keys and the constraints for their values. You can nest Collection constraints for multi-dimensional arrays. fields: Collection Required: A wrapper constraint to ensure a key is present. Required: allowExtraFields: When false, it flags any keys in the data that are not defined in the fields option. allowExtraFields: false fields allowMissingFields: When false (default), it flags any keys defined in fields that are missing from the data. Using Required provides more granular control. allowMissingFields: false fields Required Running this validation will correctly identify the missing language key and the disallowed unknown_field key. language unknown_field Custom Constraints with Service Dependencies Symfony already provides UniqueEntity in symfony/validator and symfony/doctrine-bridge. It’s usually best practice to use/extend the built-in constraint unless we need app-specific customization. Rewriting it may be unnecessary unless for non-Doctrine use cases. UniqueEntity symfony/validator symfony/doctrine-bridge namespace App\Entity; use Doctrine\ORM\Mapping as ORM; use Symfony\Component\Validator\Constraints as Assert; use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity; #[ORM\Entity] #[UniqueEntity(fields: ['email'], message: 'This email is already in use.')] class User { #[ORM\Id] #[ORM\GeneratedValue] #[ORM\Column(type: 'integer')] private ?int $id = null; #[ORM\Column(type: 'string', length: 180, unique: true)] #[Assert\NotBlank] #[Assert\Email] private string $email; #[ORM\Column(type: 'string')] #[Assert\NotBlank] private string $name; // getters/setters... } namespace App\Entity; use Doctrine\ORM\Mapping as ORM; use Symfony\Component\Validator\Constraints as Assert; use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity; #[ORM\Entity] #[UniqueEntity(fields: ['email'], message: 'This email is already in use.')] class User { #[ORM\Id] #[ORM\GeneratedValue] #[ORM\Column(type: 'integer')] private ?int $id = null; #[ORM\Column(type: 'string', length: 180, unique: true)] #[Assert\NotBlank] #[Assert\Email] private string $email; #[ORM\Column(type: 'string')] #[Assert\NotBlank] private string $name; // getters/setters... } But eventually, we’ll need validation logic that depends on other parts of your application, like checking if a username is unique in the other service. This requires a custom constraint that can access your application’s services. custom constraint The key is to create the constraint’s validator as a service and use Symfony’s dependency injection container. Let’s create an #[UniqueEntity] constraint that checks if a value for a given property already exists in a database table. #[UniqueEntity] Create the Constraint Class. This class defines the constraint’s metadata, including any options it accepts. It’s a simple data object. // src/Validator/Constraint/UniqueEntity.php namespace App\Validator\Constraint; use Symfony\Component\Validator\Constraint; #[\Attribute(\Attribute::TARGET_PROPERTY | \Attribute::TARGET_METHOD | \Attribute::IS_REPEATABLE)] class UniqueEntity extends Constraint { public string $message = 'The value "{{ value }}" is already in use.'; public string $entityClass; public string $field; public function __construct(string $entityClass, string $field, string $message = null, array $groups = null, mixed $payload = null) { $this->entityClass = $entityClass; $this->field = $field; parent::__construct([], $groups, $payload); $this->message = $message ?? $this->message; } public function getRequiredOptions(): array { return ['entityClass', 'field']; } } // src/Validator/Constraint/UniqueEntity.php namespace App\Validator\Constraint; use Symfony\Component\Validator\Constraint; #[\Attribute(\Attribute::TARGET_PROPERTY | \Attribute::TARGET_METHOD | \Attribute::IS_REPEATABLE)] class UniqueEntity extends Constraint { public string $message = 'The value "{{ value }}" is already in use.'; public string $entityClass; public string $field; public function __construct(string $entityClass, string $field, string $message = null, array $groups = null, mixed $payload = null) { $this->entityClass = $entityClass; $this->field = $field; parent::__construct([], $groups, $payload); $this->message = $message ?? $this->message; } public function getRequiredOptions(): array { return ['entityClass', 'field']; } } Create the Validator as a Service. This is where the magic happens. The validator class will perform the actual check. We inject the EntityManagerInterface to perform the database query. // src/Validator/Constraint/UniqueEntityValidator.php namespace App\Validator\Constraint; use Doctrine\ORM\EntityManagerInterface; use Symfony\Component\Validator\Constraint; use Symfony\Component\Validator\ConstraintValidator; use Symfony\Component\Validator\Exception\UnexpectedTypeException; class UniqueEntityValidator extends ConstraintValidator { public function __construct(private readonly EntityManagerInterface $entityManager) { } public function validate(mixed $value, Constraint $constraint): void { if (!$constraint instanceof UniqueEntity) { throw new UnexpectedTypeException($constraint, UniqueEntity::class); } // Ignore null and empty values to allow other constraints (e.g., NotBlank) to handle them. if (null === $value || '' === $value) { return; } $repository = $this->entityManager->getRepository($constraint->entityClass); $existingEntity = $repository->findOneBy([$constraint->field => $value]); if (null !== $existingEntity) { $this->context->buildViolation($constraint->message) ->setParameter('{{ value }}', $value) ->addViolation(); } } } // src/Validator/Constraint/UniqueEntityValidator.php namespace App\Validator\Constraint; use Doctrine\ORM\EntityManagerInterface; use Symfony\Component\Validator\Constraint; use Symfony\Component\Validator\ConstraintValidator; use Symfony\Component\Validator\Exception\UnexpectedTypeException; class UniqueEntityValidator extends ConstraintValidator { public function __construct(private readonly EntityManagerInterface $entityManager) { } public function validate(mixed $value, Constraint $constraint): void { if (!$constraint instanceof UniqueEntity) { throw new UnexpectedTypeException($constraint, UniqueEntity::class); } // Ignore null and empty values to allow other constraints (e.g., NotBlank) to handle them. if (null === $value || '' === $value) { return; } $repository = $this->entityManager->getRepository($constraint->entityClass); $existingEntity = $repository->findOneBy([$constraint->field => $value]); if (null !== $existingEntity) { $this->context->buildViolation($constraint->message) ->setParameter('{{ value }}', $value) ->addViolation(); } } } In this example, I used the EntityManagerInterface for demonstration purposes. In a real-world scenario, if we were planning to use Symfony Validator for verification here, it could also involve calls to an external API or to a synchronous message bus to fetch objective data from a distributed system. Symfony’s autoconfiguration typically handles UniqueEntityValidator automatically. If you need to register it manually or use a different naming scheme, you would add the validator.constraint_validator tag in services.yaml. validator.constraint_validator services.yaml Now we can apply our new attribute to any DTO or Entity property. // In a DTO, for example src/Dto/UserRegistrationDto.php use App\Entity\User; use App\Validator\Constraint as AppAssert; class UserRegistrationDto { #[AppAssert\UniqueEntity(entityClass: User::class, field: 'email')] #[Assert\NotBlank] #[Assert\Email] public readonly ?string $email; // ... other properties } // In a DTO, for example src/Dto/UserRegistrationDto.php use App\Entity\User; use App\Validator\Constraint as AppAssert; class UserRegistrationDto { #[AppAssert\UniqueEntity(entityClass: User::class, field: 'email')] #[Assert\NotBlank] #[Assert\Email] public readonly ?string $email; // ... other properties } When this DTO is validated, our UniqueEntityValidator service will be instantiated, the EntityManager will be injected, and it will query the database to ensure the email is unique. This pattern is incredibly flexible and is the recommended way to handle any validation logic that requires external services. Conclusion The Symfony Validator component is a cornerstone of robust application development, but its true power lies beyond simple form validation. By mastering techniques like DTO validation, you can create secure and decoupled APIs. With Group Sequences, you can model complex, state-dependent validation flows with clarity and precision. Using #[Valid] and #[Collection], you can confidently handle intricate nested data structures. And by building custom constraints as services, you can integrate any business logic — from database checks to third-party API calls — directly into your validation layer. Group Sequences #[Valid] #[Collection] custom constraints as services By embracing these advanced patterns, you can write cleaner, more expressive, and significantly more maintainable code, ensuring data integrity across every layer of your Symfony application. I’d love to hear your thoughts in comments! Stay tuned — and let’s keep the conversation going.