public function getParent($filter = null)
    {
        $parentID = $this->owner->ParentID;
        if (empty($parentID)) {
            return null;
        }
        $baseClass = $this->owner->baseClass();
        $idSQL = $this->owner->getSchema()->sqlColumnForField($baseClass, 'ID');
        return DataObject::get_one($baseClass, [
            [$idSQL => $parentID],
            $filter
        ]);
    }