class TypedDataRelationship

class TypedDataRelationship extends RelationshipBase {
  
  /**
   * {@inheritdoc}
   */
  public function getRelationship() {
    $plugin_definition = $this->getPluginDefinition();
    $data_type = $plugin_definition['data_type'];
    if (strpos($data_type, 'entity:') === 0) {
      $context_definition = new EntityContextDefinition($data_type, $plugin_definition['label']);
    }
    else {
      $context_definition = new ContextDefinition($data_type, $plugin_definition['label']);
    }
    $context_value = NULL;
    // If the 'base' context has a value, then get the property value to put on
    // the context (otherwise, mapping hasn't occurred yet and we just want to
    // return the context with the right definition and no value).
    if ($this->getContext('base')
      ->hasContextValue()) {
      $data = $this->getData($this->getContext('base'));
      $property = $this->getMainPropertyName($data);
      $context_value = $data->get($property)
        ->getValue();
    }
    $context_definition->setDefaultValue($context_value);
    return new Context($context_definition, $context_value);
  }
  
  /**
   *
   */
  public function getName() {
    return $this->getPluginDefinition()['property_name'];
  }
  
  /**
   *
   */
  protected function getData(ContextInterface $context) {
    /** @var \Drupal\Core\TypedData\ComplexDataInterface $base */
    $base = $context->getContextValue();
    $name = $this->getPluginDefinition()['property_name'];
    $data = $base->get($name);
    // @todo add configuration to get N instead of first.
    if ($data instanceof ListInterface) {
      $data = $data->first();
    }
    if ($data instanceof DataReferenceInterface) {
      $data = $data->getTarget();
    }
    return $data;
  }
  
  /**
   *
   */
  protected function getMainPropertyName(FieldItemInterface $data) {
    return $data->getFieldDefinition()
      ->getFieldStorageDefinition()
      ->getMainPropertyName();
  }
  
  /**
   *
   */
  public function getRelationshipValue() {
    $property = $this->getMainPropertyName();
    /** @var \Drupal\Core\TypedData\ComplexDataInterface $data */
    $data = $this->getRelationship()
      ->getContextData();
    $data->get($property)
      ->getValue();
  }

}