farm-2.x-dev/modules/core/parent/farm_parent.module

<?php

/**
 * @file
 * Contains farm_parent.module.
 */

use Drupal\Core\Entity\EntityTypeInterface;

/**
 * Implements hook_entity_base_field_info().
 */
function farm_parent_entity_base_field_info(EntityTypeInterface $entity_type) {

  // Add parent base field to all asset types.
  $fields = [];
  if ($entity_type->id() == 'asset') {
    $parent_info = [
      'type' => 'entity_reference',
      'label' => t('Parents'),
      'description' => t('Reference parent assets to create a lineal/hierarchical relationship.'),
      'target_type' => 'asset',
      'multiple' => TRUE,
      'weight' => [
        'form' => 40,
        'view' => 0,
      ],
    ];
    $fields['parent'] = \Drupal::service('farm_field.factory')->baseFieldDefinition($parent_info);

    // Add entity_reference_validators constraints to parent field.
    // See entity_reference_validators_entity_base_field_info_alter.
    $fields['parent']->addConstraint('CircularReference', ['deep' => TRUE]);
    $fields['parent']->addConstraint('DuplicateReference');
  }
  return $fields;
}