Zend Framework: Method "getDbTable" does not exist and was not trapped in __call()

I've implemented the guestbook example from the reference guide, and am now trying to extend it with edit functionality.

I've added this to the GuestbookController:

    public function editAction()
{
    $model    = new Application_Model_Guestbook();

    $request = $this->getRequest();
   if ($request->isPost()) { 
       Print_r('posted');die();
   }
    $model = Application_Model_GuestbookMapper::find($this->getRequest()->getParam('id'), $model);
    $form = new Application_Form_Guestbook(array('model' => $model));

    $this->view->form = $form;
}

And have altered the view index to provide a link to the edit action:

<?php foreach ($this->entries as $entry): ?>
    <?php $url = $this->url(
         array(
            'controller' => 'guestbook',
            'action'     => 'edit',
            'id'=>$entry->id
        ),
        'default',
        true) 
    ?>
<dt><?php echo $this->escape($entry->email) ?></dt>
<dd><a href="<?php echo $url; ?>"><?php echo $this->escape($entry->comment) ?></a></dd>
<?php endforeach ?>

The mapper is standard - straight copy and paste from the reference guide.

When I click thr开发者_如何学运维ough (http://quickstart/guestbook/edit/id/1) I am confronted with an exception: [Method "getDbTable" does not exist and was not trapped in __call() ]

getDbTable() is called by the find() function which is in the same file. Why then can it not find getDbTable()?

I'm running: PHP Version 5.3.3-7+squeeze1, Zend Engine v2.3.0, Apache/2.2.16 (Debian)

your class Application_Model_GuestbookMapper needs method getDbTable()