publish checkbox in list not working.

[klein-artur]

Hi,

I´m lerning with this tutorial and I found something I could not find out. In the list in Admin area we add a published control:

<?php echo JHtml::_('jgrid.published', $row->published, $i, 'helloworlds.', true, 'cb'); ?>

But when I click on that I get the following error:
Call to undefined method HelloWorldModelHelloWorlds::publish()

could it be that you forgot something to mention in step 10? Because you also write as an header:

Adding a model and modifying the existing one

But you are never modifying the existing one.