Reversing an EntityReference direction in Drupal Migrate

gistfile1.php

<?php

/**
 * This is the content type which should have the EntityReference field in the new site.
 */
class MainContentTypeMigration extends DrupalNode6Migration {
  public function __construct(array $arguments) {
    parent::__contstruct($arguments);
    
    $this->addFieldMapping('my_new_field', 'my_old_field');
    // This is how you normally handle a NodeReference migration.
    $this->addFieldMapping('normal_entityreference', 'normal_nodereference')
      ->sourceMigration(array('ExampleUnrelated'));
  }
}

/**
 * This is an unrelated content type used for the 'ExampleUnrelated' migration as an example of a normal NodeReference migration.
 */
class ExampleUnrelatedMigration extends DrupalNode6Migration {
  public function __construct(array $arguments) {
    $this->addFieldMapping('unrelated_new_field', 'unrelated_old_field');
  }
}

/**
 * This is the content type which had the NodeReference field in the old site.
 */
class ReferencedContentTypeMigration extends DrupalNode6Migration {
  public function __construct($arguments) {
    parent::__construct($arguments);
    
    $this->addFieldMapping('my_random_new_field', 'my_random_old_field');
  }
  
  // Here's where the magic sauce happens. This method is run after the content type is imported into the new site.
  // We have to do the reversal here since we can't make an EntityReference unless the referenced entity exists.
  public function complete($entity, stdClass $row) {
    // Instead of calling the sourceMigration() method above, we have to manually invoke handleSourceMigration() in order to 
    // get the node id for the node referenced on the old content type.
    $main_content_type_id = parent::handleSourceMigration('MainContent', $row->field_backward_nodereference);
    // Once we have the id for the node we want to have the reference, we can add the reference and save the node.
    $main_content = node_load($main_content_type_id);
    if (isset($main_content->nid)) {
      $main_content->field_fixed_nodereference[$main_content->language][] = array(
        'target_id' => $entity->vid,
      );
      node_save($main_content);
    }
  }
}