Silverstripe – has_many- and many_many-relations when duplicating pages

silverstripe_duplicate_relations.php

<?php 

public function duplicate() {
 
  $items_to_duplicate = array(
    'Images',
    'Somehting'
  );
 
  $page = parent::duplicate();
 
  //duplicate has many items
  foreach ($this->has_many() as $key => $className) {
    if (in_array($key, $items_to_duplicate)) {
      foreach ($this->{$key}() as $item) {
        $newField = $item->duplicate();
        $id = get_class($this) . 'ID';
        $newField->{$id} = $page->ID;
        $newField->write();
      }
    }
  }
 
  //set the relation to many_many items on created page
  foreach ($this->many_many() as $key => $className) {
    if (in_array($key, $items_to_duplicate)) {
     $page->{$key}()->addMany($this->{$key}()->getIdList());
    }
  }
 
  return $page;
}