How to use ajax to do validation fail handling?

[eluser]ted wong[/eluser]
previously, I do this:

Code:

if ($this->form_validation->run() == FALSE) //not success
{
$this->create();  
}

But I would like to return as an ajax this time. How can I get back the error message and inject to the form?? Thank you.

[eluser]ted wong[/eluser]
plz delete. I find that it is a common question. Sorry for not googling before I ask .