code tunes

As I wrote some time ago in the article about Rails, Ajax and jQuery, sometimes there are problems with Rails not interpreting correctly content type headers of ajax requests. It’s because not all web browsers send that header in the same way. 

What I proposed was to sort the request.accepts array (array containing content type headers sent by browser) so that xml content type would be the first element. That would then trigger format.xml in our respond_to block.

However that approach does not work in Rails 2.2.2, because now the request.accepts array is frozen and it cannot be modified. I spent some time googling for the solution, but with no effect. So I dived into the API and Rails’ source code and came up with pretty nice and simple solution to the problem.

class ApplicationController < ActionController::Base
  before_filter :xhr_to_xml
 
  def xhr_to_xml
    request.format = :xml if request.xhr?
  end
end

This piece of code is an equivalent of the snippet I proposed in the article I referred to at the beginning. Now all ajax request will trigger format.xml in respond_to blocks.

The more ajaxified application, the more fun it is to use. But it is also more painful do develop. What is written below is my approach to pairing Rails and Ajax. It’s a mix of tips I found over the net on blogs and forums. I use jQuery for JavaScript, but I don’t use jRails or any JS/Ajax helper methods provided by Rails. Note that all Javascript/HTML code presented here can be used even if you dont use Rails or Ruby as your web development platform. Let’s begin.

Rails is RESTful

Thanks to Rails’ RESTfulness the only thing to take care of server side is setting proper response in controllers’ actons.

class PostsController < ActionController::Base
  def index
    @posts = Post.find :all
    respond_to do |format|
      format.html
      format.xml { render :xml => @posts.to_xml }
    end
  end
end

Rails decides which format block to call basing on routes defined in routes.rb file (map.connect ‘:controller/:action/:id.:format’) and accept headers sent with request by the client.

In most cases we want Ajax requests to trigger format.xml blocks in our controllers’ actions, so we need to set proper accept headers. Let’s do it just once with application-wide setting.

// All ajax requests will trigger the format.xml block
// of +respond_to do |format|+ declarations
$.ajaxSetup({
  'beforeSend': function(xhr) {xhr.setRequestHeader("Accept", "text/xml")}
});

Browsers’ quirks

There is something worth noting here, a problem I had once with IE and Safari. The code above may work differently in various browsers. Browser set text/html accept header by default. Here IE and Safari will append text/xml to it so you’ll get something like ‘text/html; text/xml’, while Firefox will replace text/html with text/xml and you’ll get ‘text/xml’ only. This is very important because Rails will take the first format it detects in accept header and trigger respective block in controller’s action, which will be html for IE and Safari. Here’s a fix for this that shifts application/xml (if it is present) to the beggining of accept headers array.

class ApplicationController < ActionController::Base
  before_filter :correct_safari_and_ie_accept_headers
 
  def correct_safari_and_ie_accept_headers
    request.accepts.sort!{ |x, y| y.to_s == 'application/xml' ? 1 : -1 } if request.xhr?
  end
end

Ajaxify your links

Here’s a quick way to ajaxify your existing links. Add this JavaScript to your application.js file.

jQuery(document).ready(function() {
  // All A tags with class 'get', 'post', 'put' or 'delete' will perform an ajax call
  jQuery('a.get').livequery('click', function() {
    var link = jQuery(this);
    $.get(link.attr('href'), function(data) {
      if (link.attr('ajaxtarget'))
        jQuery(link.attr('ajaxtarget')).html(data);
    });
    return false;
  }).attr("rel", "nofollow");
 
  jQuery('a.post').livequery('click', function() {
    var link = jQuery(this);
    $.post(jQuery(this).attr('href'), "_method=post", function(data) {
      if (link.attr('ajaxtarget'))
        jQuery(link.attr('ajaxtarget')).html(data);
    });
    return false;
  }).attr("rel", "nofollow");
 
  jQuery('a.put').livequery('click', function() {
    var link = jQuery(this);
    $.post(jQuery(this).attr('href'), "_method=put", function(data) {
      if (link.attr('ajaxtarget'))
        jQuery(link.attr('ajaxtarget')).html(data);
    });
    return false;
  }).attr("rel", "nofollow");
 
  jQuery('a.delete').livequery('click', function() {
    var link = jQuery(this);
    $.post(jQuery(this).attr('href'), "_method=delete", function(data) {
      if (link.attr('ajaxtarget'))
        jQuery(link.attr('ajaxtarget')).html(data);
    });
    return false;
  }).attr("rel", "nofollow");
 
  jQuery('a.get, a.post, a.put, a.delete').removeAttr('onclick');
});

Just add a CSS class .get, .post, .delete, or .put to a link to make turn it into an ajax-link. I recommend you use LiveQuery plugin which will automatically bind click events to new links that appear on the page (loaded with Ajax call for-example). You can optionally set ajaxtarget attibute of the link. It expects a selector of a container in which you want to place the response.

link_to 'my cool article', article_path(@article), :class => 'get', :ajaxtarget => '#article_container'

Ajaxify your forms

For this you’d need jQuery Form Plugin.

  jQuery('form.ajax').livequery('submit', function() {
    jQuery(this).ajaxSubmit();
    return false;
  });

Now all your forms that have “ajax” class will be submitted via Ajax.

<form class="ajax">
  ...
</form>

CSRF and authenticity token

Rails has built-in protection from cross-site request forgery attacks. It relies on an authenticity token which Rails look for when dealing with POST, PUT or DELETE requests, so this token needs to be sent by the browser together with the request. The token is automatically added as a hidden field to any form you create with form_for method, it is also attached to links that have :method param set to :post, :put or :delete. In fact the token is added dynamically by Javascript code placed in link’s onclick attribute. However in one of code snippets above we stripped that onclick attribute from links to prevent the page reload after we click the link. Now we need to attack that token ourselves. First we will alter our application layout:

<head>
  <% if protect_against_forgery? %>
    <script type='text/javascript'>
    //<![CDATA[
      window._auth_token_name = "#{request_forgery_protection_token}";
      window._auth_token = "#{form_authenticity_token}";
    //]]>
    </script>
  <% end %>
</head>

Now we need to ensure that the token is sent together with ajax requests.

jQuery(document).ready(function() {
  // All non-GET requests will add the authenticity token
  // if not already present in the data packet
  jQuery("body").bind("ajaxSend", function(elm, xhr, s) {
    if (s.type == "GET") return;
    if (s.data && s.data.match(new RegExp("\\b" + window._auth_token_name + "="))) return;
    if (s.data) {
      s.data = s.data + "&";
    } else {
      s.data = "";
      // if there was no data, $ didn't set the content-type
      xhr.setRequestHeader("Content-Type", s.contentType);
    }
    s.data = s.data + encodeURIComponent(window._auth_token_name)
                    + "=" + encodeURIComponent(window._auth_token);
  });
});

We’re done, we have our ajax requests protected from CSRF attacks.

Modifing page after Ajax calls

Standard way to do page modification after Ajax call is to use Javascript code that inserts content returned by the call somewhere on the page. The other method is to put the modifying code in views that are returned by the server and just execute it in the browser. For this I’d recommend another jQuery plugin - Taconite. As the author says: “The jQuery Taconite Plugin allows you to easily make multiple DOM updates using the results of a single AJAX call. It processes an XML command document that contain instructions for updating the DOM”. Thanks to this you can for example easily use flash messages in your Ajax views.

Let this be a part of your usual layout:

<div id="flash_notice" class="flash"<%= ' style="display: none"' unless flash[:notice] %>><%= flash[:notice]  %></div>

Now let this be your taconite layout you’d use when returning views for Ajax requests:

<taconite>
  <hide select="#flash_notice" />
  <% if flash[:notice] %>
    <replaceContent select="#flash_notice">
      <%= flash[:notice] %>
    </replaceContent>
    <fadeIn select="#flash_notice" arg1="slow" />
  <% end %>
  <%= yield %>
</taconite>

This will display flash notice messages with fade-in effect after Ajax requests. Similarly you can update other elements of the page.

What’s in your toolbox?

I would love to hear from you on how you deal with Ajax in your web applications. What libraries/plugins do you use?

Users often encounter situations when they want to perform an action on a web site and get redirected to login page. Application’s task is to take care of performing the requested action right after user’s successful log in. It is easy job, unless the action is requested via ajax.

Below I present a simple solution to this problem. Let me stress the word ’simple’, because I’m not 100% satisfied with this. It works fine, however, and I didn’t have time to think about something prettier.

Here’s users_controller.php file which defines login and ajax_login actions. The former one is supposed to take care of logging the user in the system, the latter only redirects to login page via ajax response.

// users_controller.php
class UsersController extends AppController {
  var $name = 'Users';      
 
  function login() {
    //… check login credentials
    // let's assume they were ok, then perform the action requested by the user before logging in
    if ($this->Session->check('loginRedirectUrl')) {
      if ($this->Session->check('loginRedirectParams')) {
        $params = $this->Session->read('loginRedirectParams');
        $params[] = 'return';
        $this->Session->del('loginRedirectParams');
        $this->requestAction($params['url']['url'], $params);
      }
      $url = $this->Session->read('loginRedirectUrl');
      $this->Session->del('loginRedirectUrl');
      $this->redirect($url);
    }
  }      
 
  function ajax_login() {
    $this->render('ajax_login', 'ajax');
  }      
 
  /**
   * action that is accessed through ajax request
   */
  function ajax_action() {
    $this->checkSession();
    // do something creative
  }
}

Now create view file: ajax_login.ctp. It will contain JavaScript code that will make the browser load login page. That is something I don’t feel fully comfortable with, because you must handle ajax response in your view files to place this code in page body from where the browser can read it and execute. Since most ajax requests handles their responses, this will work fine, but probably there are some that don’t. You should try something else on such occasions.

// ajax_login.ctp
<script type="text/javascript">
window.location = '<?php echo 'http://'.$_SERVER['HTTP_HOST'].$html->url('users/login').'"'; ?>';
</script>

The last piece of equation is app_controller.php file where we place a method that checks wheter the user is logged in and redirects him to login page if he’s not. Call this method at the beginning of every action/method that is restricted to registered users only.

// app_controller.php
class AppController extends Controller {
  var $components = array('Session', 'RequestHandler');      
 
  function checkSession() {
    if (!$this->Session->check('User')) { // when the user is not logged in
      if ($this->RequestHandler->isAjax()) {
        $this->Session->write('loginRedirectUrl', $this->referer());
        $this->Session->write('loginRedirectParams', $this->params);
        $this->requestAction('/users/ajax_login');
      } else {
        $this->Session->write('loginRedirectUrl', $this->params['url']['url']);
        $this->redirect('/users/login');
      }
    }
  }
}

That’s it. It works both for ajax and usual requests, CakePHP 1.1 and 1.2. I just can’t figure out how to avoid this JS trick, if you have any ideas please let me know.