SS - Parent/Child GridField Relations

<?php

class ClientProjectCategory extends DataObject {

    private static $db = array(
            'Title'                         => 'Varchar(32)',
            'SortOrder'                     => 'Int',
            'URLSegment'                    => 'Varchar(32)',
            'Description'                   => 'HTMLText'
    );

    private static $has_one = array(
            'ParentCategory'    => 'ClientProjectCategory'
    );

    private static $default_sort = array(
            'SortOrder',
    );

    public function getCMSFields(){
        $fields = parent::getCMSFields();
        $fields->addFieldToTab('Root.Main', DropdownField::create('ParentCategoryID', 'Parent Category')
            ->setSource($this->ClientCategoriesForDropdown()->map('ID', 'Title'))
            ->setEmptyString(' -No Parent- '), 'Description');
        $fields->addFieldToTab('Root.Main', GridField::create('ClientProjectCategory', 'SubCategories', $this->getAllChildren(@$this->ID), GridFieldConfig_RelationEditor::create()));
        return $fields;
    }

    public function getAllChildren( $currentCategoryID = null ){
        if( $currentCategoryID == null ){
            return ClientProjectCategory::get()->filter('ParentCategoryID', -1);
        }
        $resultCategories = new ArrayList();
        $childrenCategories = ClientProjectCategory::get()->filter('ParentCategoryID', $currentCategoryID);
        if( $childrenCategories->exists() ){
            $resultCategories->merge($childrenCategories->toArray());
            foreach( $childrenCategories as $childCategory ){
                $resultCategories->merge($this->getAllChildren($childCategory->ID));
            }
        } else {
            return $childrenCategories;
        }
        return $resultCategories;
    }
}