for blogpost about error handling

ajax.js

$.ajax({
  dataType: "json",
  url: "/stuff/" + id + "/do_something",
  data: {x: x, y: y}
}).done(function(response){
  window.alert("Yay!");
}).fail(function(error){
  // even if no message is provided we should tell a user that something went wrong
  errorMessage = (error.responseJSON && error.responseJSON.message) || "Some error occurred!"
  window.alert(errorMessage);
})

handle_exceptions_1.rb

def handle_exceptions *exceptions
  # if nothing is passed then handle the most basic error class Exception
  exceptions << Exception if exceptions.empty?
  begin
    yield # execute code passed into the method
  rescue *exceptions => exception # look for specified exceptions
    # depending on the request format we should perform different actions
    respond_to do |format|
      error_message = exception.respond_to?(:message) ? exception.message : "Some error occurred!"
      format.json do
        render json: { # pass exception's name and a message
                 exception: exception.class.name,
                 message: error_message
               }, status: 400
      end
      format.html do 
        # if this request came from some page then redirect back
        # if no request referrer specified then redirect to user's home page
        path = (request.referer && :back) || root_path_for(current_user)
        redirect_to path, alert: error_message
      end
      format.js do
        # if this was Javascript request then do a simplete window.alert
        # window.alert can be replaced with more sophisticated code
        render inline: "window.alert(#{error_message.to_json})".html_safe
      end
    end
  end
end

handle_exceptions_2.rb

def handle_exceptions *exceptions
  # if nothing is passed then handle the most basic error class Exception
  exceptions << Exception if exceptions.empty?
  # if string was passed let's consider it a redirect path
  path = exceptions.select{|e| e.is_a? String}.first
  exceptions.reject!{|e| e.is_a? String} 
  begin
    yield # execute code passed into the method
  rescue *exceptions => exception # look for specified exceptions
    # depending on the request format we should perform different actions
    respond_to do |format|
      error_message = exception.respond_to?(:message) ? exception.message : "Some error occurred!"
      # if it's going to help our javascript code let's pass some additional data
      data = exception.respond_to?(:data) ? exception.data : {}
      format.json do
        render json: { # pass exception's name and a message
                 exception: exception.class.name,
                 data: data
                 message: error_message
               }, status: 400
      end
      format.html do 
        # if this request came from some page then redirect back
        # if no request referrer specified then redirect to user's home page
        if path.nil?
          path = (request.referer && :back) || root_path_for(current_user)
        end
        redirect_to path, alert: error_message
      end
      format.js do
        # if this was Javascript request then do a simplete window.alert
        # window.alert can be replaced with more sophisticated code
        render inline: "window.alert(#{error_message.to_json})".html_safe
      end
    end
  end
end

# I always forget if it's errors or exceptions, so let's make it respond to both
alias_method :handle_errors, :handle_exceptions

usage.rb

# controller
class StuffController < ApplicationController
  def some_action
    handle_exceptions UserError, stuff_page_path(params[:id]) do
      stuff = Stuff.find(params[:id])
      stuff.do_something(params[:x], params[:y])
      render json: {stuff: stuff.in_json}, status: 201 }
    end
  end
end

# model

class Stuff < ActiveRecord::Base
  def do_something x, y
    if x != y
      raise UserError.new("X has to equal Y!")
    end
  end
end

user_error.rb

class UserError < StandardError
  attr_accessor :data
  def initialize message, data = nil
    @data = data
    super(message)
  end  
end