Rails set mobile format.

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
  
  before_filter :redirect_mobile_device
  before_filter :set_mobile_format
  
  private
  
  def redirect_mobile_device
    if is_mobile_device?
      redirect_to_mobile_subdomain
    end
  end
  
  def is_mobile_device?
    if session[:preferred_format]
      session[:preferred_format] != "html"
    else
      request.user_agent =~ /iPod|iPhone|iOS/
    end
  end
  
  def redirect_to_mobile_subdomain
    redirect_to request.protocol + "mobile." + request.host_with_port + request.fullpath
  end
  
  
  def set_mobile_format
    if has_mobile_subdomain?
      request.format = :mobile
    end
  end
  
  def has_mobile_subdomain?
    request.subdomains.first == "mobile"
  end
  
end

mime_type.rb

Mime::Type.register_alias "text/html", :mobile

pages_controller_test.rb

require 'test_helper'

class PagesControllerTest < ActionController::TestCase
  # Replace this with your real tests.
  test "should set mobile format for requests to mobile subdomain" do
    @request.host = "mobile.test.host"
    get :index
    assert_equal :mobile, @request.format.to_sym
  end
  
  test "should not set mobile format for requests without mobile subdomain" do
    get :index
    assert_equal :html, @request.format.to_sym
  end

  test "should not redirect to mobile subdomain for a non mobile device" do
    get :index
    assert_response :success
  end
  
  test "should redirect to mobile subdomain for a mobile device" do
    @request.user_agent = "iPhone"
    get :index
    assert_response :redirect
    assert_redirected_to :host => "mobile.test.host"
  end
  
  test "should not redirect to mobile subdomain for mobile device with preference for full site" do
    @request.user_agent = "iPhone"
    @request.session[:preferred_format] = "html"
    get :index
    assert_response :success
  end
  
end

session_controller.rb

class SessionController < ApplicationController
  def update_preferred_format
    session[:preferred_format] = params[:preferred_format]
    redirect_to :root
  end
end