Zend 1简单聊天框

I am trying to create a simple ZF 1 chatbox application, but i'm getting an internal server 500 error when trying to insert chat posts into a db. Also the data from an ajax is being send as text/html even though i it defined to be json. As the firebug's network tab shows, i get in a response my whole html code. Could somebody please help me. Here is the controller's code:

class AjaxController extends Zend_Controller_Action
{

public function init(){
    $contextSwitch = $this->_helper->getHelper('AjaxContext');
    $contextSwitch->addActionContext('index','json')
                   ->initContext();     
}

public function indexAction()
{
   if($this->getRequest()->isXmlHttpRequest()) {
        $data = $this->getRequest()->getPost();
        $chatModel = new Application_Model_DbTable_Chat();
        $chatModel->createPost($data);

        $chatPosts = $chatModel->fetchAll();

        return $this->_helper->json->sendJson($chatPosts);
    }
 }
}

here is the ajax:

$(document).ready(function(){
$('#chat').submit(function(){
    var message = $('#message').val();
    $.ajax({
              type: 'POST',
              dataType: 'json',
              url: '/ajax/index',

              data: { 'message': message },
              success: function(data) {
                $('#outputArea').html(data);
              },
              failure: function(){ alert("does not work"); 
              }
           });  
return false;
  });                  
});

the model:

class Application_Model_DbTable_Chat extends Zend_Db_Table_Abstract
{

protected $_name = 'chat';
public function createPost($data){
    $insertData = array(
        'messages'  =>  $data['message']
    );
    return $this->insert($insertData);
}

}

and a form(which I have written manually in a layout file):

<form id="chat" action="/ajax/index" method="post">
    <h2>Chat</h2>
    <p id="outputArea"></p>
    <textarea name="message" id="message" cols="31" rows="3"></textarea>
    <input type="submit" value="post a message" name="submit" />
</form>