Drupal 8 migrate menu links with node references

migrate_plus.migration.oht_d7_menu_links.yml

langcode: en
status: true
dependencies: {  }
id: oht_d7_menu_links
class: null
field_plugin_method: null
cck_plugin_method: null
migration_tags:
  - 'Drupal 7'
  - Content
migration_group: d7
label: 'Menu links'
source:
  plugin: menu_link
  constants:
    bundle: menu_link_content
process:
  id: mlid
  bundle: constants/bundle
  title: link_title
  description: description
  menu_name:
    -
      plugin: migration_lookup
      migration: oht_d7_menu
      source: menu_name
    -
      plugin: skip_on_empty
      method: row
  link_path_processed:
    -
      plugin: static_map
      bypass: true
      map:
        '<nolink>': 'route:<nolink>'
      source: link_path
    - plugin: migrated_path_lookup
  link/uri:
    plugin: link_uri
    validate_route: false
    source:
      - '@link_path_processed'
  link/options: options
  route:
    plugin: route
    source:
      - '@link_path_processed'
      - options
  route_name: '@route/route_name'
  route_parameters: '@route/route_parameters'
  url: '@route/url'
  options: '@route/options'
  external: external
  weight: weight
  expanded: expanded
  enabled: enabled
  parent:
    plugin: menu_link_parent
    source:
      - plid
      - '@menu_name'
      - parent_link_path
  changed: updated
destination:
  plugin: 'entity:menu_link_content'
  no_stub: true
migration_dependencies:
  required:
    - oht_d7_menu
  optional:
    - oht_node_api
    - oht_node_article
    - oht_taxonomy_term

MigratedPathLookup.php

<?php

namespace Drupal\oht_migrate\Plugin\migrate\process;

use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\migrate\Annotation\MigrateProcessPlugin;
use Drupal\migrate\Plugin\migrate\process\MigrationLookup;
use Drupal\migrate\Plugin\MigratePluginManagerInterface;
use Drupal\migrate\Plugin\MigrationInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\Row;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides a 'MigratedPathLookup' migrate process plugin.
 *
 * @MigrateProcessPlugin(
 *  id = "migrated_path_lookup"
 * )
 */
class MigratedPathLookup extends ProcessPluginBase implements ContainerFactoryPluginInterface {
  /**
   * The migration plugin manager.
   *
   * @var \Drupal\migrate\Plugin\MigratePluginManagerInterface
   */
  protected $migrationPluginManager;

  /**
   * The migration to be executed.
   *
   * @var \Drupal\migrate\Plugin\MigrationInterface
   */
  protected $migration;

  /**
   * Constructs a MigrationLookup object.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
   *   The Migration the plugin is being used in.
   * @param \Drupal\migrate\Plugin\MigratePluginManagerInterface $migration_plugin_manager
   *   The Migration Plugin Manager Interface.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, MigratePluginManagerInterface $migration_plugin_manager) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->migrationPluginManager = $migration_plugin_manager;
    $this->migration = $migration;
  }

  /**
   * Creates an instance of the plugin.
   *
   * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
   *   The container to pull out services used in the plugin.
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin ID for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   *
   * @return static
   *   Returns an instance of this plugin.
   */
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $migration,
      $container->get('plugin.manager.migrate.process')
    );
  }

  /**
   * {@inheritdoc}
   * @throws \Drupal\migrate\MigrateSkipProcessException
   * @throws \Drupal\Component\Plugin\Exception\PluginException
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $matches = [];
    if (preg_match('/^(node|taxonomy\/term)\/(\d+)$/', $value, $matches)) {
      $id = $matches[2];
      $base_path = $matches[1];

      switch ($base_path) {
        case 'taxonomy/term':
          $migration_ids = [
            'oht_taxonomy_term'
          ];
          break;
        case 'node':
          $migration_ids = [
            'oht_node_api',
            'oht_node_article',
          ];
          break;
        default:
          $migration_ids = [];
      }

      $config = [
        'migration' => $migration_ids,
      ];

      /** @var MigrationLookup|bool $migration_lookup */
      $migration_lookup = $this->migrationPluginManager->createInstance('migration_lookup', $config, $this->migration);
      if ($migration_lookup) {
        $migrated_id = $migration_lookup->transform($id, $migrate_executable, $row, $destination_property);
        if ($migrated_id) {
          $value = $base_path . '/' . $migrated_id;
        }
      }
    }
    return $value;
  }


}

I found this really useful, but in case anyone runs into difficulties, the following:

link/uri:
  plugin: link_uri
  validate_route: false
  source:
    - '@link_path_processed'

Should be:

link/uri:
  plugin: link_uri
  validate_route: false
  source: '@link_path_processed'

This is because the link_uri process plugin expects a string and not an array.