Zend：如何在模型中直接访问DB

resources.db.adapter = PDO_MYSQL
resources.db.params.host = localhost
resources.db.params.username = root
resources.db.params.password = bar
resources.db.params.dbname = foo

class MyModel
{
    private $db;

    public function __construct(Zend_Db $db)
    {
        $this->db = $db;
    }

$db = $this->getInvokeArg(\'bootstrap\')->getResource(\'db\');

Zend_Registry

$db = Zend_Registry::get(\'db\');