Indexing and viewing external content.

So far, this is what I have in hopes this might help some else start and continue building an external datasource plugin for the search API. The properties are specific for what I was accessing but I do think they can be generalized. The problem I am currently having is the search API tries to load the original entity which of course doesn't exist in any drupal known datasource. Extending the ComplexDataType and setting the originalEntity would was the recommendation I found but I am little stuck on how to do that successfully.

code so far

<?php

use Drupal\Core\TypedData\ComplexDataInterface;
use Drupal\Core\TypedData\DataDefinition;
use Drupal\search_api\Datasource\DatasourcePluginBase;

/**
 * Datasource plugin for external content.
 *
 * @SearchApiDatasource(
 *   id = "external_data",
 *   label = @Translation("External Data Source"),
 *   description = @Translation("Loads external data to the drupal search index.")
 * )
 */
class ExternalDataSource extends DatasourcePluginBase {

  /**
   * {@inheritdoc}
   */
  public function getItemId(ComplexDataInterface $item) {
    return $item->getValue();
  }

  /**
   * {@inheritdoc}
   */
  public function getPropertyDefinitions() {
    $fields = $this->getFieldDefinitions();
    $properties = [];
    foreach ($fields as $field_id => $field_definition) {
      $properties[$field_id] = new DataDefinition($field_definition);
    }
    return $properties;
  }

  /**
   * Get all fields for the external data source.
   *
   * @return array
   *   Array of field definitions.
   */
  public function getFieldDefinitions() {
    return [
      'book_id' => [
        'label' => 'book_id',
        'description' => 'FT Book Id.',
        'type' => 'string',
        'prefix' => 's',
      ],
      'book_title' => [
        'label' => 'Book title',
        'description' => 'Title of the book',
        'type' => 'string',
      ],
      'book_content' => [
        'label' => 'Books content',
        'description' => 'Content about the book',
        'type' => 'string',
      ],
      'book_title_string' => [
        'label' => 'Book title string',
        'description' => 'String version of the book',
        'type' => 'string',
      ],
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function load($id) {
    $items = $this->loadMultiple(array($id));
    return $items ? reset($items) : NULL;
  }

  /**
   * {@inheritdoc}
   */
  public function loadMultiple(array $ids) {
    return parent::loadMultiple($ids);
  }

  /**
   * {@inheritdoc}
   */
  public function getItemLabel(ComplexDataInterface $item) {
    return NULL;
  }

  /**
   * {@inheritdoc}
   */
  public function getItemLanguage(ComplexDataInterface $item) {
    return 'en';
  }

  /**
   * {@inheritdoc}
   */
  public function getItemBundle(ComplexDataInterface $item) {
    return $this->getPluginId();
  }

  /**
   * {@inheritdoc}
   */
  public function getItemUrl(ComplexDataInterface $item) {
    return NULL;
  }

  /**
   * {@inheritdoc}
   */
  public function getViewModes($bundle = NULL) {
    return array();
  }

  /**
   * {@inheritdoc}
   */
  public function getBundles() {
    return array(
      $this->getPluginId() => $this->label(),
    );
  }

  /**
   * {@inheritdoc}
   */
  public function viewItem(ComplexDataInterface $item, $view_mode, $langcode = NULL) {
    return array();
  }

  /**
   * {@inheritdoc}
   */
  public function viewMultipleItems(array $items, $view_mode, $langcode = NULL) {
    $build = array();
    foreach ($items as $key => $item) {
      $build[$key] = $this->viewItem($item, $view_mode, $langcode);
    }
    return $build;
  }

}

This is the source from which I started. Its rather old but is how I got this started -> https://www.drupal.org/node/2701729

@drunken monkey thanks for the reply, I had a feeling (based on some of the other similar issues) that extending the complexDataType class would be a potential solution. I did tried the second suggestion providing all the fields in the data set but it would always fail at getOriginalObject call or some reference to a search API specific field. There is also an issue with access checking that I am not sure how I can solve. Mainly wanted to comingle both external and internal (Drupal made) content. Which based on what I am seeing for access checks would also need to be taken care of. I am trying to work through extending the complexDataType but have been running into issues there. Continuing to work through. Again thanks for the suggestions, hopefully I can get this working at least in my specific case then can throw it back out to become more generalized.