Get New Entity ID for Migrated Entity Based on Old Entity ID in Migration Map

GetMigratedNodeNewNID.php

<?php 

/**
 * Helper function to fetch the new Drupal 7 ID of a migrated entity
 * if we know the old id
 *
 * @param $old_nid - old nid of biography node from legacy node ref
 * @return tid of the new term
 */

function GetMigratedNodeNewNID($old_nid) {
  if (NULL !== $old_nid) {
    $migration = MigrationBase::getInstance('MigrationName');
    $migration_table = $migration->getMap()->getMapTable();
    $results = db_select($migration_table, 'mt')
      ->fields('mt', array('sourceid1', 'destid1'))
      // In this case the $old_nid was an array and we need the 
      // first value
      ->condition('sourceid1', $old_nid[0], '=')
      ->execute()
      ->fetchAllAssoc('sourceid1');
    if (NULL !== $results) {
      $dest_nid = $results[$new_nid][0]->destid1;
    }
  }