entity_reference_uuid-8.x-1.x-dev/tests/modules/entity_reference_uuid_test/entity_reference_uuid_test.module

<?php

/**
 * @file
 * Module to provide test views and entity types.
 */

use Drupal\node\NodeInterface;

/**
 * Implements hook_ENTITY_TYPE_presave().
 */
function entity_reference_uuid_test_node_presave(NodeInterface $entity) {
  if ($entity->bundle() === 'test_nodetype_chemical') {
    $constituents = (array) $entity->field_chemical_constituents->getValue();
    if (!isset($constituents[0]) || $constituents[0]['target_uuid'] !== $entity->uuid()) {
      // This is a dirty hack to force a self-reference to exist as the first
      // reference.
      $value = [
        'target_uuid' => $entity->uuid(),
      ];
      array_unshift($constituents, $value);
      $entity->field_chemical_constituents->setValue($constituents);
    }
    if ($entity->field_chemical_constituents->count() > 1) {
      $entity->field_chemical_mixture_flag = TRUE;
    }
    else {
      $entity->field_chemical_mixture_flag = FALSE;
    }
  }
}