安全地覆盖Magento核心资源方法

我不得不改变Magento的Mage_Catalog_Model_Resource_Eav_Mysql4_Category类的方法。我已经改变:

public function getChildrenCategories($category)
{
        $collection = $category->getCollection();
        /* @var $collection Mage_Catalog_Model_Resource_Eav_Mysql4_Category_Collection */
        $collection->addAttributeToSelect('url_key')
            ->addAttributeToSelect('name')
            ->addAttributeToSelect('all_children')
            ->addAttributeToSelect('is_anchor')
            ->addAttributeToFilter('is_active', 1)
            ->addIdFilter($category->getChildren())
            ->setOrder('position', 'ASC')
            ->joinUrlRewrite()
            ->load();
        return $collection;
}

:

public function getChildrenCategories($category)
{
            $collection = $category->getCollection();
            /* @var $collection Mage_Catalog_Model_Resource_Eav_Mysql4_Category_Collection */
            $collection->addAttributeToSelect('url_key')
                ->addAttributeToSelect('name')
                ->addAttributeToSelect('all_children')
                ->addAttributeToSelect('thumbnail')
                ->addAttributeToSelect('is_anchor')
                ->addAttributeToFilter('is_active', 1)
                ->addIdFilter($category->getChildren())
                ->setOrder('position', 'ASC')
                ->joinUrlRewrite()
                ->load();
            return $collection;
}

基本上添加了->addAttributeToSelect('thumbnail')，因为我需要获得该属性。

我现在卡住了如何安全地覆盖这个函数。我已经将整个类复制到local > Mage > Catalog > Model > Resouce > Eav > Mysql4 > Category.php中，但实际上我只需要重写那个函数。

我该怎么做?