class KeyValueEntityStorage

class KeyValueEntityStorage extends EntityStorageBase {
  
  /**
   * Length limit of the entity ID.
   */
  const MAX_ID_LENGTH = 128;
  
  /**
   * The key value store.
   *
   * @var \Drupal\Core\KeyValueStore\KeyValueStoreInterface
   */
  protected $keyValueStore;
  
  /**
   * The UUID service.
   *
   * @var \Drupal\Component\Uuid\UuidInterface
   */
  protected $uuidService;
  
  /**
   * The language manager.
   *
   * @var \Drupal\Core\Language\LanguageManagerInterface
   */
  protected $languageManager;
  
  /**
   * Constructs a new KeyValueEntityStorage.
   *
   * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
   *   The entity type.
   * @param \Drupal\Core\KeyValueStore\KeyValueStoreInterface $key_value_store
   *   The key value store.
   * @param \Drupal\Component\Uuid\UuidInterface $uuid_service
   *   The UUID service.
   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
   *   The language manager.
   * @param \Drupal\Core\Cache\MemoryCache\MemoryCacheInterface $memory_cache
   *   The memory cache.
   */
  public function __construct(EntityTypeInterface $entity_type, KeyValueStoreInterface $key_value_store, UuidInterface $uuid_service, LanguageManagerInterface $language_manager, MemoryCacheInterface $memory_cache) {
    parent::__construct($entity_type, $memory_cache);
    $this->keyValueStore = $key_value_store;
    $this->uuidService = $uuid_service;
    $this->languageManager = $language_manager;
    // Check if the entity type supports UUIDs.
    $this->uuidKey = $this->entityType
      ->getKey('uuid');
  }
  
  /**
   * {@inheritdoc}
   */
  public static function createInstance(ContainerInterface $container, EntityTypeInterface $entity_type) {
    return new static($entity_type, $container->get('keyvalue')
      ->get('entity_storage__' . $entity_type->id()), $container->get('uuid'), $container->get('language_manager'), $container->get('entity.memory_cache'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function doCreate(array $values = []) {
    // Set default language to site default if not provided.
    $values += [
      $this->getEntityType()
        ->getKey('langcode') => $this->languageManager
        ->getDefaultLanguage()
        ->getId(),
    ];
    $entity_class = $this->getEntityClass();
    $entity = new $entity_class($values, $this->entityTypeId);
    // @todo This is handled by ContentEntityStorageBase, which assumes
    //   FieldableEntityInterface. The current approach in
    //   https://www.drupal.org/node/1867228 improves this but does not solve it
    //   completely.
    if ($entity instanceof FieldableEntityInterface) {
      foreach ($entity as $name => $field) {
        if (isset($values[$name])) {
          $entity->{$name} = $values[$name];
        }
        elseif (!array_key_exists($name, $values)) {
          $entity->get($name)
            ->applyDefaultValue();
        }
        unset($values[$name]);
      }
    }
    return $entity;
  }
  
  /**
   * {@inheritdoc}
   */
  public function doLoadMultiple(array $ids = NULL) {
    if (empty($ids)) {
      $entities = $this->keyValueStore
        ->getAll();
    }
    else {
      $entities = $this->keyValueStore
        ->getMultiple($ids);
    }
    return $this->mapFromStorageRecords($entities);
  }
  
  /**
   * {@inheritdoc}
   */
  public function loadRevision($revision_id) {
    return NULL;
  }
  
  /**
   * {@inheritdoc}
   */
  public function deleteRevision($revision_id) {
    return NULL;
  }
  
  /**
   * {@inheritdoc}
   */
  public function doDelete($entities) {
    $entity_ids = array_keys($entities);
    $this->keyValueStore
      ->deleteMultiple($entity_ids);
  }
  
  /**
   * {@inheritdoc}
   */
  public function save(EntityInterface $entity) {
    $id = $entity->id();
    if ($id === NULL || $id === '') {
      throw new EntityMalformedException('The entity does not have an ID.');
    }
    // Check the entity ID length.
    // @todo This is not config-specific, but serial IDs will likely never hit
    //   this limit. Consider renaming the exception class.
    if (strlen($entity->id()) > static::MAX_ID_LENGTH) {
      throw new ConfigEntityIdLengthException("Entity ID {$entity->id()} exceeds maximum allowed length of " . static::MAX_ID_LENGTH . ' characters.');
    }
    return parent::save($entity);
  }
  
  /**
   * {@inheritdoc}
   */
  protected function doSave($id, EntityInterface $entity) {
    $is_new = $entity->isNew();
    // Save the entity data in the key value store.
    $this->keyValueStore
      ->set($entity->id(), $entity->toArray());
    // If this is a rename, delete the original entity.
    if ($this->has($id, $entity) && $id !== $entity->id()) {
      $this->keyValueStore
        ->delete($id);
    }
    return $is_new ? SAVED_NEW : SAVED_UPDATED;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function has($id, EntityInterface $entity) {
    return $this->keyValueStore
      ->has($id);
  }
  
  /**
   * {@inheritdoc}
   */
  public function hasData() {
    return (bool) $this->keyValueStore
      ->getAll();
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getQueryServiceName() {
    return 'entity.query.keyvalue';
  }

}