Attempt rolling additional tracking.

... didn't seem to work.

embargo.services.yml

@@ -31,3 +31,7 @@ services:
       - '@url_generator'
     tags:
       - { name: 'event_subscriber' }
+  embargo.tracking_event_subscriber:
+    class: Drupal\embargo\EventSubscriber\TrackingEventSubscriber
+    tags:
+      - { name: 'event_subscriber' }

src/EventSubscriber/TrackingEventSubscriber.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\embargo\EventSubscriber;
+
+use Drupal\search_api\Event\MappingForeignRelationshipsEvent;
+use Drupal\search_api\Event\SearchApiEvents;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Inform search_api of entity_reference structures.
+ */
+class TrackingEventSubscriber implements EventSubscriberInterface {
+
+  /**
+   * {@inheritDoc}
+   */
+  public static function getSubscribedEvents() : array {
+    return [
+      SearchApiEvents::MAPPING_FOREIGN_RELATIONSHIPS => 'foreignRelationshipMap',
+    ];
+  }
+
+  /**
+   * Inform search_api of our computed entity_reference listings.
+   *
+   * @param \Drupal\search_api\Event\MappingForeignRelationshipsEvent $event
+   *   The event by which to inform.
+   */
+  public function foreignRelationshipMap(MappingForeignRelationshipsEvent $event) : void {
+    $mapping =& $event->getForeignRelationshipsMapping();
+    $index = $event->getIndex();
+    foreach ($index->getDatasources() as $id => $datasource) {
+      if (!in_array($datasource->getEntityTypeId(), ['file', 'media', 'node'])) {
+        continue;
+      }
+
+      $mapping[] = [
+        'datasource' => $id,
+        'entity_type' => 'embargo',
+        'property_path_to_foreign_entity' => 'embargo:entity:id',
+        'field_name' => 'id',
+      ];
+    }
+  }
+
+}