How to add a custom validation for the custom entity?

Hello! In this article I would like to describe how we can add custom validation

for our custom entity type.

Let’s use a ´our_custom_module_name´ module, as example, to create an entity validation,
which will check if we have the same entity already (with the same title).

Inside the module folder we should create a constraint:

Constraints in Drupal 8 are also plugins which usually hold a small amount of information about how data is actually being validated, what error message should be used in case of failure and any additional options the validator needs.

/our_custom_module_name/src/Plugin/Validation/Constraint/ExistsEntityWithNameConstraint.php

<?php
/**
 * @file
 * Contains \Drupal\our_custom_module_name\Plugin\Validation\Constraint\ExistsEntityWithNameConstraint.
 */

namespace Drupal\our_custom_module_name\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;


/**
 * Constraint for checking if a default entity already exists.
 *
 * @Constraint(
 * id = "ExistsEntityWithName",
 * label = @Translation("Entity exists", context = "Validation"),
 * type = { "entity" }
 * )
 */
class ExistsEntityWithNameConstraint extends Constraint {

 /**
 * Message shown when the entity already exists.
 *
 * @var string
 */
 public $messageExists = 'Entity %entity already exists';
}

Next, we should create a validator class, where all our validation logic should be placed:

The validator class (which is referenced by the constraint) is responsible for checking the data.

/our_custom_module_name/src/Plugin/Validation/Constraint/ExistsEntityWithNameConstraintValidator.php

<?php
/**
 * @file
 * Contains \Drupal\our_custom_module_name\Plugin\Validation\Constraint\ExistsEntityWithNameConstraintValidator.
 */

namespace Drupal\our_custom_module_name\Plugin\Validation\Constraint;

use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
use Drupal\Core\Entity\EntityTypeManager;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * Validates the ExistsEntityWithNameConstraint.
 */
class ExistsEntityWithNameConstraintValidator extends ConstraintValidator implements ContainerInjectionInterface {

  /**
   * Validator 2.5 and upwards compatible execution context.
   *
   * @var \Symfony\Component\Validator\Context\ExecutionContextInterface
   */
  protected $context;

  /**
   * @var \Drupal\Core\Entity\EntityTypeManager
   */
  protected $entityTypeManager;

  /**
   * Constructs a new ExistsEntityWithNameConstraintValidator.
   *
   * @param \Drupal\Core\Entity\EntityTypeManager
   *   The user storage handler.
   */
  public function __construct(EntityTypeManager $entity_type_manager) {
    $this->entityTypeManager = $entity_type_manager;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container->get('entity_type.manager'));
  }

  /**
   * {@inheritdoc}
   *
   * Here we validate entities and check if the same entity
   * (with the same name) already exists.
   */
  public function validate($entity, Constraint $constraint) {
    $name = $entity->name->value;

    if (isset($name)) {
      $entity_storage = $this->entityTypeManager->getStorage($entity->bundle());
      // Check if we already have the same entity.
      $ex_entity = $entity_storage->loadByProperties(array('name'=> $name));
      if (!empty($ex_entity) && is_array($ex_entity)) {
        // Takes entity id.
        $ex_entity_id = array_shift($ex_entity)->id();
        // If we just update the same entity, then skip validation.
        if ($entity->id() != $ex_entity_id) {
          $this->context->buildViolation($constraint->messageExists, array(
            '%entity' => $name
          ))
            ->atPath('name')
            ->addViolation();
        }
      }
    }
  }
}

And last step. We should add annotation to our custom entity to use this validation:

...
constraints = {
 "ExistsEntityWithName" = {}
}
...