模型类如何获得资源模型得实例

假设有如下前提:
[color=darkred]1. etc/config.xml有如下配置[/color]

        <global>
            <models>
                <hotel>
                    <class>Cartz_Hotel_Model</class>
                    <resourceModel>hotel_mysql4</resourceModel>
                </hotel>
                <hotel_mysql4>
                    <class>Cartz_Hotel_Model_Mysql4</class>
                    <entities>
                         <room>
                            <table>rooms</table>
                        </room>
                    </entities>
                </hotel_mysql4>
            </models>
            <resources>
                <hotel_setup>
                    <setup>
                        <module>Cartz_Hotel</module>
                    </setup>
                    <connection>
                        <use>core_setup</use>
                    </connection>
                </hotel_setup>
                <hotel_write>
                    <connection>
                        <use>core_write</use>
                    </connection>
                </hotel_write>
                <hotel_read>
                    <connection>
                        <use>core_read</use>
                    </connection>
                </hotel_read>
            </resources>
         </global>

[color=darkred]2. 模型类为model/Room.php[/color]

    <?php
    class Cartz_Hotel_Model_Room extends Mage_Core_Model_Abstract{
        protected function _construct() {
            $this->_init('hotel/room');
        }
        public function listing(){
            $this->_getResource()->findAll();
        }
    }
    ?>

[color=darkred]3. 相应的资源模型类是model/Mysql4/Room.php[/color]

  <?php
    class Cartz_Hotel_Model_Mysql4_Room extends Mage_Core_Model_Mysql4_Abstract{
       protected function _construct(){
          $this->_init('hotel/room', 'id');
       }
       public function findAll() {
          $handle = $this->_getWriteAdapter();
          $query  = $handle->query('select name from rooms');
          while ($row = $query->fetch()) {
                $row = new Varien_Object($row);
             echo $row->getName() . "<br/>";
            }
       }
    }
    ?>

这段代码昭示了两个知识点:
1. model类的方法_getResource()获得相应资源模型类的实例。
2. 在资源模型类中, 方法 _getWriteAdapter()用来获得数据库的连接句柄