Ruby on Rails: Easy way to render a very flexible 404 page

We often find ourselves needing to render a 404 page from Rails whenever something cannot be found. There are many different ways to do this, but so far this is the one I have found to give me the easiest time of it:

Use a render 404 action in ApplicationController

I normally add some version of this to ApplicationController in order to render the 404 error from there. This way I can use a completely normal Rails template file. In this case a slim template.

class ApplicationController < ActionController::Base

  # ...
  
  def error_not_found
    render '/application/errors/404', status: :not_found, formats: [ :html ]
  end
  
  # ...

end

Note: This assumes your 404.html.erb template is in app/views/application/errors/404.html.erb

Add a catch-all route to Rails routes

Now we add the following to the very end of our routes.rb file:

match '*unmatched', to: 'application#error_not_found', via: :all

With just these 4 lines, we will catch all unmatched requests in any format and be able to render them just like any other page. This enables us to do things like:

• Our 404 page can use the same layout as the rest of the application, instead of being “special”.
• Track the referrer of 404 errors, to find out if they are coming from bad internal or external links.
• Add page suggestions so we have a chance of redirecting users to where they might want to go.
  • Add specific page suggestions based on where the user came from / what it looks like they are trying to go to

However, when we combine this with rescue_from ActiveRecord::RecordNotFound, with: :error_not_found we get a powerful way of redirecting users to our 404 page:

Redirect ActiveRecord::RecordNotFound errors to 404 page

If we rescue from ActiveRecord::RecordNotFound, we can add a way to very easily add “could not find that” redirections to our controllers:

So we use find_by! instead of find_by when appropriate, and expand our ApplicationController with a rescue_from:

class ApplicationController < ActionController::Base

  rescue_from ActiveRecord::RecordNotFound, with: :error_not_found
  
  # ...
  
  def error_not_found
    render '/application/errors/404', status: :not_found
  end
  
  # ...

end

Caveat: Careful with performance

While this makes things very easy for us, the above is also harder on performance than just a plain /public/404.html file. I have rarely found this to be much of an issues, but be sure to keep your error_not_found action lean and fast, and add caching where / if necessary.