Can I create non-default EntityReferenceSelection?

Question

I have the following EntityReferenceSelection plugin:

/**
 * Provides an entity reference selection for my_entity.
 *
 * @EntityReferenceSelection(
 *   id = "extra:my_entity",
 *   label = "...",
 *   entity_types = {
 *     "my_entity"
 *   },
 *   group = "extra",
 *   weight = 1
 * )
 */
class MyEntityExtraSelection extends DefaultSelection {

  protected function getAllowedIds() {
    return [...];
  }

  protected function buildEntityQuery($match = NULL, $match_operator = 'CONTAINS') {
    $query = parent::buildEntityQuery($match, $match_operator);
    $query->condition('id', $this->getAllowedIds(), 'IN');
    return $query;
  }

  public function validateReferenceableNewEntities(array $entities) {
    $entities = parent::validateReferenceableNewEntities($entities);
    $allowed = $this->getAllowedIds();
    return array_filter($entities, function ($entity) {
      return in_array($entity->id(), $allowed);
    });
  }

}

I'd like to call it extra:my_entity because I don't want this filtering to occur by default, every time I use it, only when I refer to it using the 'handler' => 'extra' setting explicitely.

But the core plugin manager gives these errors:

Warning: uasort() expects parameter 1 to be array, null given in Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManager->getPluginId() (line 65 of core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginManager.php).
Warning: end() expects parameter 1 to be array, null given in Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManager->getPluginId() (line 66 of core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginManager.php).
Warning: key() expects parameter 1 to be array, null given in Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManager->getPluginId() (line 67 of core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginManager.php).

Answer 1

As commented, you should not need to provide a deriver for a single entity reference selection plugin. A random plugin ID like extra_my_entity should work. The only condition is that you define the (in this case irrelevant) group property with the same value:

/**
 * Provides an entity reference selection for my_entity.
 *
 * @EntityReferenceSelection(
 *   id = "extra_my_entity",
 *   label = "...",
 *   entity_types = {
 *     "my_entity"
 *   },
 *   group = "extra_my_entity",
 *   weight = 1
 * )
 */
class MyEntityExtraSelection extends DefaultSelection {

See Entity reference selection plugins break when not following a weird ID pattern.

When you extend DefaultSelection then also extend the schema:

config/schema/mymodule.schema.yml

entity_reference_selection.extra_my_entity
  type: entity_reference_selection.default
  label: '...'

---

Starting a new group

A group in contrast consists of multiple plugin classes. A base class that derives the installed entity types using for example the base plugin ID extra. On installation Drupal will scan the entity types and create for each entity a plugin ID: extra:[entity_type]. You can override the base class by using this plugin ID syntax and the group extra. These group members then don't need to define a deriver, like the core plugins you've seen overriding default:*.

It's not a good idea to combine base and group member in one class, because then deriving the plugin ID as base ends up in a:b:b and other unpredicted things can happen.

Answer 2

Update

I needed it again, this time already in Drupal 10. The old tricks are no longer necessary, it works without the deriver and the extra key now.

Earlier

Finally, after some investigation, two required items never documented:

• unlike the core EntityReferenceSelection plugins that, for some reason, get away without it, you have to provide

deriver = "Drupal\Core\Entity\Plugin\Derivative\DefaultSelectionDeriver"

in the plugin annotation.

• It might be completely unnecessary in your case but you have to make sure your custom entity type has an entity_key named label. If it doesn't, core will use PhpSelector instead of your selector, forcedly.