Migrate process plugin: entity_lookup

EntityLookup.php

<?php

/**
 * @file
 * Contains \Drupal\migrate_entity_lookup\Plugin\migrate\process\EntityLookup.
 */

namespace Drupal\migrate_entity_lookup\Plugin\migrate\process;

use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;
use Drupal\migrate\MigrateException;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\Core\Entity\Query\QueryFactory;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;


/**
 * This plugin changes the current value by using it for an entity property search
 * and returning an entity id.
 *
 * @MigrateProcessPlugin(
 *   id = "entity_lookup"
 * )
 */
class EntityLookup extends ProcessPluginBase implements ContainerFactoryPluginInterface {

  /**
   * The entity type manager.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected $entityQuery;

  /**
   * Constructs an EntityLookup 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\Core\Entity\Query\QueryFactory $entity_query
   *   The entity query factory.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, QueryFactory $entity_query) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->entityQuery = $entity_query;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('entity.query')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if (empty($value)) {
      // @todo: allow a default_value if nothing in source.
      return null;
    }
    else {
      $query = $this->entityQuery->get($this->configuration['entity_type']);
      $entity_ids = $query->condition($this->configuration['property'], $value)->range(0, 1)->execute();
      if (count($entity_ids)) {
        return reset($entity_ids);
      }
      else {
        // @todo: Return a default value if failed to retrieve one.
        return null;
      }
    }
  }
}

migrate.migration.SNIPPET.yml

process:
  field_dest:
    plugin: entity_lookup
    source: source_field
    entity_type: taxonomy_term
    property: name