chx

    // When looking up the destination ID we require an array with both the
    // source key and value, e.g. ['nid' => 41]. In this case, $source_id_values need to be ordered the same
    // order as $this->sourceIdFields(). However, the Migration process
    // plugin doesn't currently have a way to get the source key so we presume
    // the values have been passed through in the correct order.
    if (!isset($source_id_values[0])) {
      $source_id_values = array_intersect_key($this->sourceIdFields(), $source_id_values);
    }