JSON API response for 'No route matches'

In your routes.rb, add the last line

MyAppApi::Application.routes.draw do
  # omitted lines
  match '*a', :to => 'errors#routing'
end

NOTE: The "a" is actually a parameter in the Rails 3 Route Globbing technique. For example, if your url was /some-random-path, then params[:a] equals "some-random-path". But notice the preceding slash is not there. To mock the actual path, I used request.path instead.

Create errors_controller.rb that inherits from application_controller.rb

class ErrorsController < ApplicationController
  def routing
    not_found(ActionController::RoutingError.new("No route matches [#{request.method}] #{request.path}"))
  end
end

The exception will throw the message similar to a typical no route message, with the method and the requested path

Then, In your application_controller.rb, add the not_found action

class ApplicationController < ActionController::Base

  rescue_from ActiveRecord::RecordNotFound, :with => :not_found

  def not_found(exception)
    respond_to do |format|
      format.json { render :json => {:error => exception.message}, :status => :not_found }
      format.any { render :text => "Error: #{exception.message}", :status => :not_found }
    end
  end
end

The client will get this json response for non-routable path

{
  error: "No route matches [GET] /groups/cf8260ab-a431-4e40-8520-a8d24c681639/join"
}