Gaffe makes having customized error pages in Rails applications an easy thing.
It takes advantage of a feature present in Rails 3.2 (and 4.0+, obviously) called exceptions_app.

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It comes with default error pages but makes it very easy to override them (which you should do). The default error pages look like this:

Installation

Add this line to your application’s Gemfile:

gem 'gaffe'

Usage

The easiest way to use Gaffe is with an initializer:

# config/initializers/gaffe.rb
Gaffe.enable!

Custom controller

However, if you want to use your own controller:

# config/initializers/gaffe.rb
Gaffe.configure do |config|
  config.errors_controller = 'ErrorsController'
end

Gaffe.enable!

It’s also possible to use a custom controller based on the URL in which the error has occured. Both absolute and relative URL supported. This is especially useful if you have an application that also serves API requests via JSON. You would probably want to serve API errors through JSON and regular errors through HTML pages.

# config/initializers/gaffe.rb
Gaffe.configure do |config|
  config.errors_controller = {
    %r[^/api/] => 'Api::ErrorsController',
    %r[^/] => 'ErrorsController',
    %r[^www.example.com] => 'HostSpecificErrorsController'
  }
end

Gaffe.enable!

The only required thing to do in your custom controller is to include the Gaffe::Errors module.

Only show will be called so you might want to override it. If you don’t override it, Gaffe will try to render the view "errors/#{@rescue_response}" within your application (or use its default error page if the view doesn’t exist).

You might also want to get rid of filters and other stuff to make sure that error pages are always accessible.

class ErrorsController < ApplicationController
  include Gaffe::Errors

  # Make sure anonymous users can see the page
  skip_before_action :authenticate_user!

  # Override 'error' layout
  layout 'application'

  # Render the correct template based on the exception “standard” code.
  # Eg. For a 404 error, the `errors/not_found` template will be rendered.
  def show
    # Here, the `@exception` variable contains the original raised error
    render "errors/#{@rescue_response}", status: @status_code
  end
end

For example, you might want your API::ErrorsController to return a standard JSON response:

class API::ErrorsController < API::ApplicationController
  include Gaffe::Errors

  # Make sure anonymous users can see the page
  skip_before_action :authenticate_user!

  # Disable layout (your `API::ApplicationController` probably does this already)
  layout false

  # Render a simple JSON response containing the error “standard” code
  # plus the exception name and backtrace if we’re in development.
  def show
    output = { error: @rescue_response }
    output.merge! exception: @exception.inspect, backtrace: @exception.backtrace.first(10) if Rails.env.development? || Rails.env.test?
    render json: output, status: @status_code
  end
end

Custom views

You can (and should!) also use your own views. You just have to create a layout:

<!-- app/views/layouts/error.html.erb -->
<h1>Error!</h1>
<%= yield %>

And create a different view for each possible error rescue response (rails reference). For example, for 404 errors:

<!-- app/views/errors/not_found.html.erb -->
<p>This page does not exist.</p>

Custom exceptions

If your application is raising custom exceptions (through gems or your code) and you want to render specific views when it happens, you can map them to specific rescue responses.

# config/application.rb
config.action_dispatch.rescue_responses.merge! 'CanCan::AccessDenied' => :forbidden
config.action_dispatch.rescue_responses.merge! 'MyCustomException' => :not_acceptable

Rails development environment

Rails prefers to render its own debug-friendly errors in the development environment, which is totally understandable. However, if you want to test Gaffe’s behavior in development you’ll have to edit the config/environments/development.rb file.

# Make Rails use `exceptions_app` in development
config.consider_all_requests_local = false

Rails test environment

You also have to configure Rails’ test environment so it lets Gaffe handle exceptions in request tests. You’ll have to edit the config/environments/test.rb file.

# Make Rails use `exceptions_app` in tests
config.consider_all_requests_local = false

# Render exceptions instead of raising them
config.action_dispatch.show_exceptions = true

Unfortunately, controller tests (called functional tests in Rails) do not work with Gaffe, since they only test method calls in the controller class — they do not go through the entire Rack stack to simulate a real HTTP request.

To test responses sent by Gaffe, you must use request tests.

Contributors

• @remiprev
• @simonprev
• @jmuheim

License

Gaffe is © 2013-2016 Mirego and may be freely distributed under the New BSD license. See the LICENSE.md file.

The mushroom cloud logo is based on this lovely icon by Gokce Ozan, from The Noun Project. Used under a Creative Commons BY 3.0 license.

About Mirego

Mirego is a team of passionate people who believe that work is a place where you can innovate and have fun. We're a team of talented people who imagine and build beautiful Web and mobile applications. We come together to share ideas and change the world.

We also love open-source software and we try to give back to the community as much as we can.