How to Create a custom Validation Constraint

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 simple 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:

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// src/Acme/DemoBundle/Validator/Constraints/ContainsAlphanumeric.php
namespace Acme\DemoBundle\Validator\Constraints;

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.';
}

Note

The @Annotation annotation is necessary for this new constraint in order to make it available for use in classes via annotations. Options for your constraint are represented as public properties on the constraint class.

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 includes some simple default logic:

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// in the base Symfony\Component\Validator\Constraint class
public function validatedBy()
{
    return get_class($this).'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 is also simple, and only has one required method validate():

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// src/Acme/DemoBundle/Validator/Constraints/ContainsAlphanumericValidator.php
namespace Acme\DemoBundle\Validator\Constraints;

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

class ContainsAlphanumericValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint)
    {
        if (!preg_match('/^[a-zA-Za0-9]+$/', $value, $matches)) {
            // If you're using the new 2.5 validation API (you probably are!)
            $this->context->buildViolation($constraint->message)
                ->setParameter('%string%', $value)
                ->addViolation();
            );

            // If you're using the old 2.4 validation API
            /*
            $this->context->addViolation(
                $constraint->message,
                array('%string%' => $value)
            );
            */
        }
    }
}

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.

2.5The buildViolation method was added in Symfony 2.5. For usage examples with older Symfony versions, see the corresponding versions of this documentation page.

Using the new Validator

Using custom validators is very easy, just as the ones provided by Symfony itself:

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    # src/Acme/BlogBundle/Resources/config/validation.yml
    Acme\DemoBundle\Entity\AcmeEntity:
        properties:
            name:
                - NotBlank: ~
                - Acme\DemoBundle\Validator\Constraints\ContainsAlphanumeric: ~
    
  • Annotations
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    // src/Acme/DemoBundle/Entity/AcmeEntity.php
    use Symfony\Component\Validator\Constraints as Assert;
    use Acme\DemoBundle\Validator\Constraints as AcmeAssert;
    
    class AcmeEntity
    {
        // ...
    
        /**
         * @Assert\NotBlank
         * @AcmeAssert\ContainsAlphanumeric
         */
        protected $name;
    
        // ...
    }
    
  • XML
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    <!-- src/Acme/DemoBundle/Resources/config/validation.xml -->
    <?xml version="1.0" encoding="UTF-8" ?>
    <constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping http://symfony.com/schema/dic/constraint-mapping/constraint-mapping-1.0.xsd">
    
        <class name="Acme\DemoBundle\Entity\AcmeEntity">
            <property name="name">
                <constraint name="NotBlank" />
                <constraint name="Acme\DemoBundle\Validator\Constraints\ContainsAlphanumeric" />
            </property>
        </class>
    </constraint-mapping>
    
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    // src/Acme/DemoBundle/Entity/AcmeEntity.php
    use Symfony\Component\Validator\Mapping\ClassMetadata;
    use Symfony\Component\Validator\Constraints\NotBlank;
    use Acme\DemoBundle\Validator\Constraints\ContainsAlphanumeric;
    
    class AcmeEntity
    {
        public $name;
    
        public static function loadValidatorMetadata(ClassMetadata $metadata)
        {
            $metadata->addPropertyConstraint('name', new NotBlank());
            $metadata->addPropertyConstraint('name', new ContainsAlphanumeric());
        }
    }
    

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 your constraint validator has dependencies, such as a database connection, it will need to be configured as a service in the dependency injection container. This service must include the validator.constraint_validator tag and an alias attribute:

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    services:
        validator.unique.your_validator_name:
            class: Fully\Qualified\Validator\Class\Name
            tags:
                - { name: validator.constraint_validator, alias: alias_name }
    
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    <service id="validator.unique.your_validator_name" class="Fully\Qualified\Validator\Class\Name">
        <argument type="service" id="doctrine.orm.default_entity_manager" />
        <tag name="validator.constraint_validator" alias="alias_name" />
    </service>
    
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    $container
        ->register('validator.unique.your_validator_name', 'Fully\Qualified\Validator\Class\Name')
        ->addTag('validator.constraint_validator', array('alias' => 'alias_name'));
    

Your constraint class should now use this alias to reference the appropriate validator:

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public function validatedBy()
{
    return 'alias_name';
}

As mentioned above, Symfony will automatically look for a class named after the constraint, with Validator appended. If your constraint validator is defined as a service, it's important that you override the validatedBy() method to return the alias used when defining your service, otherwise Symfony won't use the constraint validator service, and will instantiate the class instead, without any dependencies injected.

Class Constraint Validator

Beside validating a class property, a constraint can have a class scope by providing a target in its Constraint class:

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

With this, the validator 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->addViolationAt(
                'foo',
                $constraint->message,
                array(),
                null
            );
        }
    }
}

Note that a class constraint validator is applied to the class itself, and not to the property:

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    # src/Acme/BlogBundle/Resources/config/validation.yml
    Acme\DemoBundle\Entity\AcmeEntity:
        constraints:
            - Acme\DemoBundle\Validator\Constraints\ContainsAlphanumeric: ~
    
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    /**
     * @AcmeAssert\ContainsAlphanumeric
     */
    class AcmeEntity
    {
        // ...
    }
    
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    <!-- src/Acme/BlogBundle/Resources/config/validation.xml -->
    <class name="Acme\DemoBundle\Entity\AcmeEntity">
        <constraint name="Acme\DemoBundle\Validator\Constraints\ContainsAlphanumeric" />
    </class>
    

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