silverstripe-frameworktest/code/FrameworkTestSiteTreeExtension.php

<?php

use SilverStripe\ORM\DataExtension;
class FrameworkTestSiteTreeExtension extends DataExtension
{
    
    private static $has_one = array('RelationFieldsTestPage' => 'RelationFieldsTestPage');
    private static $belongs_many_many = array('RelationFieldsTestPages' => 'RelationFieldsTestPage');
}