Zend Framework 事务处理 有关问题

Zend Framework 事务处理 问题
官方给出一个示例，如下：

<?php

// 创建一个 $db对象, 然后开始做一个事务处理.
$db->beginTransaction();

// 尝试数据库操作.
// 假如成功,commit该操作;
// 假如, roll back.
try {
    $db->query(...);
    $db->commit();
} catch (Exception $e) {
    $db->rollBack();
    echo $e->getMessage();
}

?>

我写的实例中，确没有通过，而是提示
Fatal error: Uncaught exception 'PDOException' with message 'There is already an active transaction' in E:\php\ZendFramework\library\Zend\Db\Adapter\Pdo\Abstract.php:305 Stack trace: #0 E:\php\ZendFramework\library\Zend\Db\Adapter\Pdo\Abstract.php(305): PDO->beginTransaction() #1 E:\php\ZendFramework\library\Zend\Db\Adapter\Abstract.php(484): Zend_Db_Adapter_Pdo_Abstract->_beginTransaction() #2 D:\svnrepos\WiClub\src\application\models\shop.php(76): Zend_Db_Adapter_Abstract->beginTransaction() #3 D:\svnrepos\WiClub\src\application\controllers\ShopController.php(507): shop->inserttest(Array) #4 E:\php\ZendFramework\library\Zend\Controller\Action.php(513): ShopController->shopaddtestAction() #5 E:\php\ZendFramework\library\Zend\Controller\Dispatcher\Standard.php(289): Zend_Controller_Action->dispatch('shopaddtestActi...') #6 E:\php\ZendFramework\library\Zend\Controller\Front.php(946): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http)) #7 D:\svnrepos\WiC in E:\php\ZendFramework\library\Zend\Db\Adapter\Pdo\Abstract.php on line 305

似乎是提示已经有一个事务在运行？刚接触ZF不久，不太熟悉，把相关源文件贴上来吧

config.ini

[general]
db.adapter = PDO_MYSQL
db.config.host = localhost
db.config.username = root
db.config.password = 123456
db.config.dbname = testdb 
db.config.charset = utf8
date_default_timezone = "PRC"

Util.php    [所在文件夹 Common,与controllers文件夹，models文件夹同级]

<?php
require_once 'Zend/Config/Exception.php';
require_once 'Zend/Config/Ini.php';
require_once 'Zend/Registry.php';
require_once 'Zend/Db.php';
require_once 'Zend/Db/Table.php';
//require_once 'common/Blowfish.php';
//require_once 'common/Hosts.php';
class Common_Util
{	
        public static function getDb($configName,$setcharset=false)
	{
		if($configName)
		{
			$config=new Zend_Config_Ini('../config.ini',$configName);
                        $registry= Zend_Registry::getInstance();
                        $registry->set('config',$config);
                        $params=$config->db->config->toArray();
                        $params['options']=array(PDO::ATTR_PERSISTENT => true);
                        $db = Zend_Db::factory($config->db->adapter,$params);