How to Create a Custom Validation Constraint

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How to Create a Custom Validation Constraint

You can create a custom constraint by extending the base constraint class, Constraint. As an example you're going to create a basic validator that checks if a string contains only alphanumeric characters.

Creating the Constraint Class

First you need to create a Constraint class and extend Constraint:

  • Annotations
  • Attributes
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// src/Validator/ContainsAlphanumeric.php
namespace App\Validator;

use Symfony\Component\Validator\Constraint;

/**
 * @Annotation
 */
class ContainsAlphanumeric extends Constraint
{
    public string $message = 'The string "{{ string }}" contains an illegal character: it can only contain letters or numbers.';
    // If the constraint has configuration options, define them as public properties
    public string $mode = 'strict';
}

Add @Annotation or #[\Attribute] to the constraint class if you want to use it as an annotation/attribute in other classes.

5.2

The ability to use PHP attributes to configure constraints was introduced in Symfony 5.2. Prior to this, Doctrine Annotations were the only way to annotate constraints.

Creating the Validator itself

As you can see, a constraint class is fairly minimal. The actual validation is performed by another "constraint validator" class. The constraint validator class is specified by the constraint's validatedBy() method, which has this default logic:

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// in the base Symfony\Component\Validator\Constraint class
public function validatedBy()
{
    return static::class.'Validator';
}

In other words, if you create a custom Constraint (e.g. MyConstraint), Symfony will automatically look for another class, MyConstraintValidator when actually performing the validation.

The validator class only has one required method validate():

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// src/Validator/ContainsAlphanumericValidator.php
namespace App\Validator;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;
use Symfony\Component\Validator\Exception\UnexpectedTypeException;
use Symfony\Component\Validator\Exception\UnexpectedValueException;

class ContainsAlphanumericValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint): void
    {
        if (!$constraint instanceof ContainsAlphanumeric) {
            throw new UnexpectedTypeException($constraint, ContainsAlphanumeric::class);
        }

        // custom constraints should ignore null and empty values to allow
        // other constraints (NotBlank, NotNull, etc.) to take care of that
        if (null === $value || '' === $value) {
            return;
        }

        if (!is_string($value)) {
            // throw this exception if your validator cannot handle the passed type so that it can be marked as invalid
            throw new UnexpectedValueException($value, 'string');

            // separate multiple types using pipes
            // throw new UnexpectedValueException($value, 'string|int');
        }

        // access your configuration options like this:
        if ('strict' === $constraint->mode) {
            // ...
        }

        if (!preg_match('/^[a-zA-Z0-9]+$/', $value, $matches)) {
            // the argument must be a string or an object implementing __toString()
            $this->context->buildViolation($constraint->message)
                ->setParameter('{{ string }}', $value)
                ->addViolation();
        }
    }
}

Inside validate(), you don't need to return a value. Instead, you add violations to the validator's context property and a value will be considered valid if it causes no violations. The buildViolation() method takes the error message as its argument and returns an instance of ConstraintViolationBuilderInterface. The addViolation() method call finally adds the violation to the context.

Using the new Validator

You can use custom validators like the ones provided by Symfony itself:

  • Annotations
  • Attributes
  • YAML
  • XML
  • PHP
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// src/Entity/User.php
namespace App\Entity;

use App\Validator as AcmeAssert;
use Symfony\Component\Validator\Constraints as Assert;

class User
{
    // ...

    /**
     * @Assert\NotBlank
     * @AcmeAssert\ContainsAlphanumeric(mode="loose")
     */
    protected string $name = '';

    // ...
}

If your constraint contains options, then they should be public properties on the custom Constraint class you created earlier. These options can be configured like options on core Symfony constraints.

Constraint Validators with Dependencies

If you're using the default services.yaml configuration, then your validator is already registered as a service and tagged with the necessary validator.constraint_validator. This means you can inject services or configuration like any other service.

Create a Reusable Set of Constraints

In case you need to consistently apply a common set of constraints across your application, you can extend the Compound constraint.

5.1

The Compound constraint was introduced in Symfony 5.1.

Class Constraint Validator

Besides validating a single property, a constraint can have an entire class as its scope.

For instance, imagine you also have a PaymentReceipt entity and you need to make sure the email of the receipt payload matches the user's email. First, create a constraint and override the getTargets() method:

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// src/Validator/ConfirmedPaymentReceipt.php
namespace App\Validator;

use Symfony\Component\Validator\Constraint;

/**
 * @Annotation
 */
class ConfirmedPaymentReceipt extends Constraint
{
    public string $userDoesNotMatchMessage = 'User\'s e-mail address does not match that of the receipt';

    public function getTargets(): string
    {
        return self::CLASS_CONSTRAINT;
    }
}

Now, the constraint validator will get an object as the first argument to validate():

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// src/Validator/ConfirmedPaymentReceiptValidator.php
namespace App\Validator;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;
use Symfony\Component\Validator\Exception\UnexpectedValueException;

class ConfirmedPaymentReceiptValidator extends ConstraintValidator
{
    /**
     * @param PaymentReceipt $receipt
     */
    public function validate($receipt, Constraint $constraint): void
    {
        if (!$receipt instanceof PaymentReceipt) {
            throw new UnexpectedValueException($receipt, PaymentReceipt::class);
        }

        if (!$constraint instanceof ConfirmedPaymentReceipt) {
            throw new UnexpectedValueException($constraint, ConfirmedPaymentReceipt::class);
        }

        $receiptEmail = $receipt->getPayload()['email'] ?? null;
        $userEmail = $receipt->getUser()->getEmail();

        if ($userEmail !== $receiptEmail) {
            $this->context
                ->buildViolation($constraint->userDoesNotMatchMessage)
                ->atPath('user.email')
                ->addViolation();
        }
    }
}

Tip

The atPath() method defines the property with which the validation error is associated. Use any valid PropertyAccess syntax to define that property.

A class constraint validator must be applied to the class itself:

  • Annotations
  • Attributes
  • YAML
  • XML
  • PHP
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// src/Entity/PaymentReceipt.php
namespace App\Entity;

use App\Validator\ConfirmedPaymentReceipt;

/**
 * @ConfirmedPaymentReceipt
 */
class PaymentReceipt
{
    // ...
}

Testing Custom Constraints

Use the ConstraintValidatorTestCase` class to simplify writing unit tests for your custom constraints:

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// tests/Validator/ContainsAlphanumericValidatorTest.php
namespace App\Tests\Validator;

use App\Validator\ContainsAlphanumeric;
use App\Validator\ContainsAlphanumericValidator;
use Symfony\Component\Validator\Test\ConstraintValidatorTestCase;

class ContainsAlphanumericValidatorTest extends ConstraintValidatorTestCase
{
    protected function createValidator()
    {
        return new ContainsAlphanumericValidator();
    }

    public function testNullIsValid()
    {
        $this->validator->validate(null, new ContainsAlphanumeric());

        $this->assertNoViolation();
    }

    /**
     * @dataProvider provideInvalidConstraints
     */
    public function testTrueIsInvalid(ContainsAlphanumeric $constraint)
    {
        $this->validator->validate('...', $constraint);

        $this->buildViolation('myMessage')
            ->setParameter('{{ string }}', '...')
            ->assertRaised();
    }

    public function provideInvalidConstraints(): iterable
    {
        yield [new ContainsAlphanumeric(message: 'myMessage')];
        // ...
    }
}
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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