class UserReference

class UserReference extends ReferenceBase {
  
  /**
   * The plugin ID for the reference type migration.
   *
   * @var string
   */
  protected $userTypeMigration = 'd6_user_role';
  
  /**
   * {@inheritdoc}
   */
  protected function getEntityTypeMigrationId() {
    return $this->userTypeMigration;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function entityId() {
    return 'uid';
  }
  
  /**
   * {@inheritdoc}
   */
  public function defineValueProcessPipeline(MigrationInterface $migration, $field_name, $data) {
    $process = [
      'plugin' => 'sub_process',
      'source' => $field_name,
      'process' => [
        'target_id' => [
          'plugin' => 'migration_lookup',
          'migration' => 'd6_user',
          'source' => 'uid',
        ],
      ],
    ];
    $migration->setProcessOfProperty($field_name, $process);
  }

}