Skip to content
A gem for building mobile optimized Rails applications using semantic, media query-based device detection and server side component optimization.
Find file
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


A system for building mobile optimized Rails applications using semantic, media query-based device detection and server side progressive enhancement.


Ress is an extension of the devicejs library written by Boris Smus. It adds a back end for adapting server responses based on client side feature detection. Ress allows you to specify alternate versions of your website, along with media queries for which devices should be redirected to which version.

How it Works

HTML Annotations

When you register alternate mobile versions of your website, Ress adds annotations to the <head> of your document that describes where these pages are located and which devices should be redirected to them.

For example, a typical alternate version for a mobile site might include a tag like this:

<link rel="alternate" media="only screen and (max-width: 640px)" href="" >

The mobile version of the page would then have a link pointing back the canonical version:

<link rel="canonical" href="" >

These annotations conform to SEO best practices for mobile optimized websites as documented by Google here.

Feature Detection

Device.js will read all of the version links in your markup, and redirect you to the appropriate URL that serves the correct version of your webapp.

When a request comes into your site, ress runs a some javascript to determine if there is an alternate version available that matches the client. If there is, the user is redirected to the url for that version.

Server Side Components

Ress allows you to customize how your Rails application responds to requests in two ways:

  1. It adds controller and helper methods to detect which version of your site has been requested. This is useful for small tweeks in html or behaviour, eg:
<% if mobile_request? %>
  <%= image_tag 'low-res.png' %>
<% else %>
  <%= image_tag 'high-res.png' %>
<% end %>
  1. It prepends a view path for each alternate version of your site, so that you can override which templates or partials are rendered for certain requests. For example if you want to render a different html form for creating users on the mobile version of your site you could create app/mobile_views/users/_form.html.erb and Ress would have Rails select that template over app/views/users/_form.html.erb when a request comes in to the mobile version.


Add this line to your application's Gemfile:

gem 'ress'

And then execute:

$ bundle install

Run the installation generator:

$ rails g ress:install


Version override

You can manually override the detector and load a particular version of the site by passing in the device GET parameter with the ID of the version you'd like to load. This will look up the link tag based on the specified ID and load that version. For example, if you are on desktop but want the tablet version, visiting will redirect to the tablet version at

Relatedly, you can prevent redirection completely, by specifying the force=1 GET parameter. For example, if you are on desktop and know the URL of the tablet site, you can load

<!-- Include a way to manually switch between device types -->
    <li><a href="?device=desktop">Desktop</a></li>
    <li><a href="?device=tablet">Tablet</a></li>
    <li><a href="?device=phone">Phone</a></li>


There are a couple of Modernizr features that must be included in order for Ress's javascript feature detection to function. If you are not already using Modernizr in your application you can automatically include a build that has been packaged with the gem by setting config.include_modernizr = true in config/initializers/ress.rb. If you include your own build (recommended), make sure that it includes "Touch Events" and "Media Queries", eg:

Performance considerations

The javascript included by Ress does some checks and will use client-side redirection to point users to the right version of your webapp. Client-side redirection can have a performance overhead (though I haven't measured it). If you find this is true, you can keep your DOM the same, still using the SEO-friendly <link rel="alternate"> tags, but simply remove the ress.js script and do your own server-side UA-based pushing.

Browser support

The feature detection javascript should work in all browsers that support document.querySelectorAll. Notably, this excludes IE7. If you want it to work in IE7 and below, please include a polyfill.


Given how many browsers and devices we have these days, there are bound to be bugs. If you find them, please report them and (ideally) fix them in a pull request.

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request
Something went wrong with that request. Please try again.