Entity Validation API: Retrieve parent field entity while validating

Question

I'm using the Drupal 8 Entity Validation API. I've implemented my Constraint and ConstraintValidator classes and added them as a constraint against the relevant entity field.

If I understand the validate method signature for ConstraintValidator correctly, I'm given a list of fields ($items) and the constraint ($constraint). However, I need to retrieve the parent entity in my logic.

How can I get the parent entity while with the validation API?

Answer 1

By using FieldItemList::getEntity like in this core validator:

Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldValueValidator:

class UniqueFieldValueValidator extends ConstraintValidator {

  /**
   * {@inheritdoc}
   */
  public function validate($items, Constraint $constraint) {
    if (!$item = $items->first()) {
      return;
    }
    $field_name = $items->getFieldDefinition()->getName();
    /** @var \Drupal\Core\Entity\EntityInterface $entity */
    $entity = $items->getEntity();

The first argument of a validate method is of mixed type. For the Drupal Entity Validation API this can be a field value, a field item list or an entity, depending on where you add the constraint.