D7 WBM upgrade path

Here is a slightly modified query when you are only migration only latest revision of the node.

Below query only retrieves currently active moderation state for migration.

    $query = $source->getDatabase()
      ->select('workbench_moderation_node_history', 'wm')
      ->fields('wm', ['state'])
      ->condition('wm.nid', $row->getSourceProperty('nid'))
      ->condition('wm.is_current', '1');

    $value = $query->execute()->fetchField();
    if ($value) {
      $row->setSourceProperty('moderation_state', $value);
    }