gistfile1.rb

class ApplicationController < ActionController::Base
  protect_from_forgery

  rescue_from ActiveRecord::RecordNotFound, :with => :render_404

  # Render 404: Not Found
  def render_404(exception = nil)
    logger.info( clean_backtrace(exception, "Rendering 404 due to exception") )
    if /(jpe?g|png|gif)/i === request.path
      # Images should simply return the error as text
      return render(text: "404 Not Found", status: 404 )
    end
    render_error(404, "Resource Not Found")
  end

  protected

  def render_error(code, message)
    respond_to do |format|
      format.html { render :template => "/errors/#{code}", :layout => 'application', :status => code }
      format.json { render json: { error: message }, status: code }
      format.xml { render xml: { error: message }, status: code }
      format.any { render nothing: true, status: code }
    end
    true  # so we can do "render_xxx and return"    
  end

  # Returns a "clean" backtrace that is suitable for humans
  def clean_backtrace(exception, preamble="Caught Exception")
    "\n\n#{preamble}:" + "\n\n#{exception.class} (#{exception.message}):\n    " +
            Rails.backtrace_cleaner.clean(exception.backtrace).join("\n    ") + "\n\n"
  end
end