application_controller.rb

#app/controllers/application_controller.rb
class ApplicationController < ActionController::Base
  def opensearch
    response.headers['Content-Type'] = 'application/opensearchdescription+xml; charset=utf-8'
  end

application_controller_spec.rb

#spec/controller/application_controller_spec.rb

require 'spec_helper'

describe ApplicationController do
  describe 'GET "opensearch"' do
    before do
      xml_http_request :get, :opensearch
    end
    subject { response }
    it { should render_template('application/opensearch') }

    context 'header' do
      subject { response.header['Content-Type'] }
      it { should == 'application/opensearchdescription+xml; charset=utf-8' }
    end
  end

end

application_helper.rb

#app/helper/application_helper.rb
module ApplicationHelper
  def full_image_path_helper(img)
    root_url.chomp('/') + asset_path(img)
  end
end

opensearch.xml.builder

#app/views/application/opensearch.xml.builder
xml.instruct!
xml.OpenSearchDescription(:xmlns => 'http://a9.com/-/spec/opensearch/1.1/', 'xmlns:moz' => 'http://www.mozilla.org/2006/browser/search/') do
  xml.ShortName(t(:default_title))
  xml.InputEncoding('UTF-8')
  xml.Description(t(:opensearch_description))
  xml.Contact(t(:contact_email))
  xml.Image(full_image_path_helper('icon.png'), height: 16, width: 16, type: 'image/png')
  # escape route helper or else it escapes the '{' '}' characters. then search doesn't work
  xml.Url(type: 'text/html', method: 'get', template: CGI::unescape(search_url(q: '{searchTerms}' )))
  xml.moz(:SearchForm, root_url)
end

opensearch.xml.builder_spec.rb

#spec/view/application/opensearch.xml.builder_spec.rb
require 'spec_helper'

describe "application/opensearch" do
  before do
    render
    @xml_element = Nokogiri.XML rendered
  end
  context 'short name' do
    subject { @xml_element.search('ShortName').text }
    it { should == I18n.translate('default_title') }
  end

  context 'search description' do
    subject { @xml_element.search('Description').text }
    it { should == I18n.translate('opensearch_description') }
  end

  context 'input encoding ' do
    subject { @xml_element.search('InputEncoding').text }
    it { should == 'UTF-8' }
  end
  
  context 'contact email' do
    subject { @xml_element.search('Contact').text }
    it { should == I18n.translate('contact_email') }
  end

  context 'image' do
    subject { @xml_element.search('Image').text }
    it { should == 'http://test.host/assets/icon.png' }
  end

  context 'search terms' do
    subject { @xml_element.search('Url').attr('template').value }
    it { should == CGI.unescape(search_url(:q => '{searchTerms}')) }
  end

end