Mapping metadata to reference entities

We are also looking at this and it does not appear to be supported, so marking as a Feature Request.

So the current metadata refresh looks pretty simple. See MetadataRefreshTrait::forceMappedFieldRefresh.

We should be able to adapt that method a bit to account for entity reference fields. The problem here is that we have to register field lookups.

Consider this case:

• We have a Drupal taxonomy - "File type" that we want to sync from the DAM.
• File type contains "pdf", "doc", "image"
• In Drupal, these are referenced by entity ID
• Acquia DAM doesn't know the entity ID, it probably stores a string
• So we would need a means to lookup the proper entity reference by string

This is all pretty simple if we assume that the Entity Reference is a taxonomy term, but I am not sure that is a safe assumption.

What I would propose is adding an event to the trait which allows for a data lookup on the Drupal side before we resolve the call to $translation->set($entity_field_name, $media_source->getMetadata($translation, $metadata_attribute_name));

Something like this:

  protected function forceMappedFieldRefresh(MediaInterface $media): void {
    $media_source = $media->getSource();
    $translations = $media->getTranslationLanguages();
    foreach ($translations as $langcode => $data) {
      if ($media->hasTranslation($langcode)) {
        $translation = $media->getTranslation($langcode);
        // Try to set fields provided by the media source and mapped in
        // media type config.
        foreach ($translation->bundle->entity->getFieldMap() as $metadata_attribute_name => $entity_field_name) {
          if ($translation->hasField($entity_field_name)) {
            // New pseudo-code here.
            $metadata = $media_source->getMetadata($translation, $metadata_attribute_name);
            // We have to write this event handler, which can use $translation->getFieldDefinition($entity_field_name) to get the target type
            $metadata = $this->eventDispatcher->dispatch(new MappedFieldEvent($translation, $metadata, $entity_field_name));
            $translation->set($entity_field_name, $metadata);
          }
        }
      }
    }
  }

That should provide enough metadata to derive the entity lookup.

I would propose that we write the Event handler and a default implementation, allowing other modules / sites to also respond.

I am not going to have immediate time to pursue this, but here is a patch for testing. It does the following:

1. Moves the metadata update trait to a service
2. Introduces an event that fires after metadata has been mapped

I started to write default event handlers, but that gets tricky, so it seems "good enough" to allow developers to write their own Event handlers to respond to the metadata update.

Rerolled the patch to update according to the latest changes in AssetUpdateChecker QueueWorker.

There might be a possibility when custom implementation where we require the metadata attribute name in the event handler, i have added an extra argument for this.