Mapping metadata to reference field

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.