Incomplete PHP class when serializing object in se

I'm working on a shopping cart (Cart model). One of its protected properties is "_items", which holds an array of Product objects. They (Products) all get stored in DB for populating the session (using ZF, Zend_Session_SaveHandler_DbTable() etc.).

public function addItem(Model_Product $product, $qty)
{
    $qty = (int) $qty;
    $pId = $product->getId();

    if ($qty > 0) {
        $this->_items[$pId] = array('product' => $product, 'qty' => $qty);
    } else {
        // if the quantity is zero (or less), remove item from stack
        unset($this->_items[$pId]);
    }

    // add new info to session
    $this->persist();
}

In the controller, I grab a Product obj from DB with the ProductMapper and provide it to "addItem()":

    $product1 = $prodMapper->getProductByName('cap');
    $this->_cart->addItem($product1, 2);

getProductByName() returns a new populated Model_Product object.

I usually get the

Please ensure that the class definition "Model_Product" of the object you are trying to operate on was loaded _before_ ...

error message, a session dump obviously shows

['__PHP_Incomplete_Class_Name'] => 'Model_Product'

I know about the "declaring the class before serializing it". My problem is this: how can I declare the Product class in addItem(), if it's injected (first param) in the first place? Wouldn't a new declaration (like new Model_Product()) overwrite the param (original object) in addItem()? Must I declare it in the Cart model again?

Besides, I'll surely get a Cannot redeclare class Model_Product if I... redeclare it in Cart.

In ZF's bootstrap, the session was started before autoloading.

    /**
     * Make XXX_* classes available
     */
    protected function _initAutoloaders()
    {
        $loader = new Zend_Application_Module_Autoloader(array(
                    'namespace' => 'XXX',
                    'basePath' => APPLICATION_PATH
                ));
    }

    public function _initSession()
    {
        $config = $this->_config->custom->session;

        /**
         * For other settings, see the link below:
         * http://framework.zend.com/manual/en/zend.session.global_session_management.html
         */
        $sessionOptions = array(
            'name'             => $config->name,
            'gc_maxlifetime'   => $config->ttl,
            'use_only_cookies' => $config->onlyCookies,
//            'strict'           => true,
//            'path'             => '/',
        );

        // store session info in DB
        $sessDbConfig = array(
            'name'           => 'xxx_session',
            'primary'        => 'id',
            'modifiedColumn' => 'modified',
            'dataColumn'     => 'data',
            'lifetimeColumn' => 'lifetime'
        );

        Zend_Session::setOptions($sessionOptions);
        Zend_Session::setSaveHandler(new Zend_Session_SaveHandler_DbTable($sessDbConfig));
        Zend_Session::start();
    }

When I was getting the errors I was talking about, the method declaration was the other way around: _initSession() was first, then _initAutoloaders() - and this was the exact order ZF was processing them.

I'll test some more, but this seems to work (and logical). Thanks for all your suggestions.