Rails: How do I create a custom 404 error page that uses the asset pipeline?

For Rails 4.1 I like this answer, add an asset type better; however I have not tried it. On Rails 4.0.8, these three references helped me:

1. Dynamic error pages is the second reference in the question. This worked just fine for me.

2. Custom error pages may have cribbed from the first reference, or the other way around, but goes the extra mile by adding some information about testing with Capybara.

3. I did not do the Capybara testing because I didn’t want to change the test configuration; however, RSpec-Rails Request Specs clued me in to test these requests independently and see that they complete and return the correct content.

What follows is a nutshell description of what is taught by the three references:

1. Add the following setting to config/environments/production.rb

   # Route exceptions to the application router vs. default
   config.exceptions_app = self.routes
   

2. Edit the routing configuration, config/routes.rb to direct the error pages to an errors controller

     # error pages
     %w( 404 422 500 503 ).each do |code|
       get code, :to => "errors#show", :code => code
     end
   

   will route the 404, 422, 500, and 503 page requests to the show action of the errors controller with a parameter code that has the value of the status code.

3. Create the controller, app/controllers/errors_controller.rb. Here is the entire content:

   class ErrorsController < ApplicationController
   
     def show
       status_code = params[:code] || 500
       flash.alert = "Status #{status_code}"
       render status_code.to_s, status: status_code
     end
   
   end
   

   My preference was to set a status message on flash.alert

4. Create the pages themselves. I use .erb Here is app/views/errors/500.html.erb

   <p>Our apology.  Your request caused an error.</p>
   <%= render 'product_description' %>
   

   So you see that you can render a partial. The page renders with all of the layout boilerplate from app/views/layouts/application.html.erb or any other layout boilerplate that you have configured. That includes the <div id='alert'><%= alert %></div> that displays the status message from the flash.

5. Tested with RSpec by adding a test file, spec/requests/errors_request_spec.rb. Here is abbreviated content of that file that shows a test of the 500 status page:

   require 'rails_helper'
   
   RSpec.describe "errors", :type => :request do
   
     it "displays the 500 page" do
       get "/500"
       assert_select 'div#alert', 'Status 500'
       assert_select 'div[itemtype]'
     end
   
   end
   

   The first assertion checks for the flash alert. The second assertion checks for the partial.