Media migration

foo_media.yml

id: foo_media
label: Foo media
source:
  plugin: sd8_table
  table: migrate_map_foo_file
  ids:
    sourceid1:
      type: integer
      # The table plugin sets the alias to t and this must be specified
      # otherwise SqlBase::initializeIterator() dies with a SQL error when it
      # joins the map table for this migration which also contains fields called
      # sourceid1 and sourceid2.
      alias: t
    sourceid2:
      type: string
      alias: t
process:
  field_media_image: destid1
destination:
  plugin: entity:media
  default_bundle: image
migration_dependencies:
  required:
    - migrate_map_foo

Table.php

<?php

namespace Drupal\sd8_import\Plugin\migrate\source;

use Drupal\migrate\Plugin\migrate\source\SqlBase;

/**
 * Table source from database.
 *
 * @MigrateSource(
 *   id = "sd8_table",
 *   source_module = "sd8"
 * )
 */
class Table extends SqlBase {

  /**
   * @return \Drupal\Core\Database\Query\SelectInterface
   */
  public function query() {
    $query = $this->select($this->configuration['table'], 't')
      ->fields('t');
    foreach (array_keys($this->getIds()) as $id) {
      $query->orderBy($id);
    }
    return $query;
  }

  /**
   * {@inheritdoc}
   */
  public function getIds() {
    return $this->configuration['ids'];
  }
  
  /**
   * {@inheritdoc}
   */
  public function fields() {
    return [];
  }

}