toResourceIdentifiersWithArityRequired example

// According to the specification, you are only allowed to POST to a     // relationship if it is a to-many relationship.     /** @var \Drupal\Core\Field\EntityReferenceFieldItemListInterface $field_list */
    $field_list = $entity->{$internal_relationship_field_name};
    /** @var \Drupal\field\Entity\FieldConfig $field_definition */
    $field_definition = $field_list->getFieldDefinition();
    $is_multiple = $field_definition->getFieldStorageDefinition()->isMultiple();
    if (!$is_multiple) {
      throw new ConflictHttpException(sprintf('You can only POST to to-many relationships. %s is a to-one relationship.', $related));
    }

    $original_resource_identifiers = ResourceIdentifier::toResourceIdentifiersWithArityRequired($field_list);
    $new_resource_identifiers = array_udiff(
      ResourceIdentifier::deduplicate(array_merge($original_resource_identifiers, $resource_identifiers)),
      $original_resource_identifiers,
      [ResourceIdentifier::class, 'compare']
    );

    // There are no relationships that need to be added so we can exit early.     if (empty($new_resource_identifiers)) {
      $status = static::relationshipResponseRequiresBody($resource_identifiers, $original_resource_identifiers) ? 200 : 204;
      return $this->getRelationship($resource_type, $entity, $related, $request, $status);
    }