States to taxonomy IDs

gistfile1.php

<?php

/**
 * @file
 * Contains \Drupal\migrate_plus\Plugin\migrate\process\TermReference.
 */

namespace Drupal\migrate_plus\Plugin\migrate\process;

use Drupal\taxonomy\TermStorageInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\migrate\Entity\MigrationInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\Row;

/**
 * Looks up a taxonomy term by title.
 *
 * @MigrateProcessPlugin(
 *   id = "term_reference"
 * )
 */
class TermReference extends ProcessPluginBase implements ContainerFactoryPluginInterface {

  /**
   * {@inheritdoc}
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, TermStorageInterface $termStorage) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->migration = $migration;
    $this->termStorage = $termStorage;
  }


  /**
   * {@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('entity.manager')->getStorage('taxonomy_term')
    );
  }

  protected function getTermId($name) {
    if (!$this->terms) {
      $tids = $this->termStorage
        ->getQuery()
        ->condition('vid', 'state')
        ->execute();
      $terms = $this->termStorage->loadMultiple($tids);
      // For taxonomy, $terms = $this->termStorage->loadTree('state', 0, NULL, TRUE); works as well.
      foreach ($terms as $term) {
        // This could be $term->label() instead of $term->name->value.
        $this->terms[$term->name->value] = $term->id();
      }
    }
    return $this->terms[$name];
  }

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    return $this->getTermId($value);
  }

}