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);
    }

}