massageRelationshipInput example

if (empty($context['related']) || empty($field_definitions[$context['related']])) {
      throw new BadRequestHttpException('Invalid or missing related field.');
    }
    /** @var \Drupal\field\Entity\FieldConfig $field_definition */
    $field_definition = $field_definitions[$context['related']];
    $target_resource_types = $resource_type->getRelatableResourceTypesByField($resource_type->getPublicName($context['related']));
    $target_resource_type_names = array_map(function DResourceType $resource_type) {
      return $resource_type->getTypeName();
    }, $target_resource_types);

    $is_multiple = $field_definition->getFieldStorageDefinition()->isMultiple();
    $data = $this->massageRelationshipInput($data, $is_multiple);
    $resource_identifiers = array_map(function D$value) use ($target_resource_type_names) {
      // Make sure that the provided type is compatible with the targeted       // resource.       if (!in_array($value['type'], $target_resource_type_names)) {
        throw new BadRequestHttpException(sprintf(
          'The provided type (%s) does not match the destination resource types (%s).',
          $value['type'],
          implode(', ', $target_resource_type_names)
        ));
      }
      return new ResourceIdentifier($value['type'], $value['id'], $value['meta'] ?? []);
    },