Cannot create base_field_override entity, ID already exists

Fail only patch attached. The problem is that $row->getDestinationProperty($this->getKey('id')) doesn't return the entity id for the base_field_override since the id is not in the incoming data but calculated from entity_type, bundle and field_name.

I've added a getEntityId() method to the Entity base class and then a base_field_override destination to override the new method.

Arh, missing the new file.

It isn't only for the test. This same error happens when you migrate to Drupal 8 and already have a content type in your Drupal 6 site that exists in D8. For example, page and article.

+++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
@@ -106,7 +106,7 @@ public function fields(MigrationInterface $migration = NULL) {
+    $entity_id = $old_destination_id_values ? reset($old_destination_id_values) : $this->getEntityId($row);
     if (!empty($entity_id) && ($entity = $this->storage->load($entity_id))) {
       $this->updateEntity($entity, $row);
     }

But now it has a valid entity id it tries to update what's on the site?

Just using what is on the site that's being migrated to could be problematic. I think we need to report this to the user and ask them what to do... but this is a tricky issue.

This destination attempts to load the entity from D8 first and then update appropriately otherwise create a new one if it didn't exist. This is inline with the other destinations currently in core.

Note: We will have a "delete_existing" feature coming soon that allows the user to choose whether they want the destination to try update first or always starts a fresh.