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

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

New in version 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 $constraint)
    {
        if (!$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/AcmeEntity.php
namespace App\Entity;

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

class AcmeEntity
{
    // ...

    /**
     * @Assert\NotBlank
     * @AcmeAssert\ContainsAlphanumeric(mode="loose")
     */
    protected $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 apply some common set of constraints in different places consistently across your application, you can extend the Compound constraint.

New in version 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. You only need to add this to the Constraint class:

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

With this, the validator's validate() method gets an object as its first argument:

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class ProtocolClassValidator extends ConstraintValidator
{
    public function validate($protocol, Constraint $constraint)
    {
        if ($protocol->getFoo() != $protocol->getBar()) {
            $this->context->buildViolation($constraint->message)
                ->atPath('foo')
                ->addViolation();
        }
    }
}

Tip

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

A class constraint validator is applied to the class itself, and not to the property:

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

use App\Validator as AcmeAssert;

/**
 * @AcmeAssert\ProtocolClass
 */
class AcmeEntity
{
    // ...
}
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.