I'm trying to override a function in `Mage_CatalogRule_Model_Rule` but I can't get it to work.

**config.xml**

    <config>
        <global>
            <models>
                <rule>
                    <rewrite>
                        <rule>Apptha_Timer_Model_CatalogRule_Model_Rule</rule>
                    </rewrite>
                </rule>
            </models>
        </global>
    </config>

**/app/code/local/Apptha/Timer/Model/CatalogRule/Model/Rule.php**

    class Apptha_Timer_Model_CatalogRule_Model_Rule extends Mage_CatalogRule_Model_Rule {
        protected function _convertFlatToRecursive(array $data) {
            //not called
        }
    }

What am I missing? I've been able to override controllers and blocks just fine.


  [1]: http://inchoo.net/ecommerce/magento/overriding-magento-blocks-models-helpers-and-controllers/