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Error Checking and Debugging with Ruby on Rails

In this conclusion to a six-part series covering web development and Ruby on Rails, you'll learn how to send error messages to your email and more. This article is excerpted from chapter 15 of the Ruby Cookbook, written by Lucas Carlson and Leonard Richardson (O'Reilly, 2006; ISBN: 0596523696). Copyright 2006 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

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By: O'Reilly Media
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April 26, 2007
  1. · Error Checking and Debugging with Ruby on Rails
  2. · 15.21 Documenting Your Web Site
  3. · 15.22 Unit Testing Your Web Site
  4. · 15.23 Using breakpoint in Your Web Application

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Error Checking and Debugging with Ruby on Rails
(Page 1 of 4 )

15.20 Automatically Sending Error Messages to Your Email


You want to receive a descriptive email message every time one of your users encounters an application error.


Any errors that occur while running your application are sent to the ActionController::Base#log_error method. If you've set up a mailer (as shown in Recipe 15.19) you can override this method and have it send mail to you. Your code should look something like this:

  class ApplicationController < ActionController::Base

def log_error(exception)

That code rounds up a wide variety of information about the state of the Rails request at the time of the failure. It captures the exception object, the corresponding backtrace, the session data, the CGI request parameters, and the values of all environment variables.

The overridden log_error calls Notification.deliver_error_messsage, which assumes you've created a mailer called "Notification", and defined the method Notification.error_message. Here's the implementation:

  class Notification < ActionMailer::Base
    def error_message(exception, trace, session, params, env, sent_on = Time.now)

      @recipients    =
      @from          = 'error@mydomain.com'
      @subject       = "Error message:
      @sent_on       = sent_on
      @body = {
:exception => exception,
        :trace => trace,
        :session => session,
        :params => params,
        :env => env

The template for this email looks like this:

  <!-- app/views/notification/error_message.rhtml -->

  Time: <%= Time.now %>
  Message: <%= @exception.message %>
  Location: <%= @env['REQUEST_URI'] %> 
  Action: <%= @params.delete('action') %> </td> </tr>
  Controller: <%= @params.delete('controller') %> </td> </tr>
  Query: <%= @env['QUERY_STRING'] %> </td> </tr>
  Method: <%= @env['REQUEST_METHOD'] %> </td> </tr>
  SSL: <%= @env['SERVER_PORT'].to_i == 443 ? "true" : "false" %>
  Agent: <%= @env['HTTP_USER_AGENT'] %>

  <%= @trace.to_a.join("</p>\n<p>") %>

  <% @params.each do |key, val| -%>
  * <%= key %>: <%= val.to_yaml %>
  <% end -%>

  <% @session.each do |key, val| -%>
  * <%= key %>: <%= val.to_yaml %>
  <% end -%>

  <% @env.each do |key, val| -%>
  * <%= key %>: <%= val %>
  <% end -%>


ActionController::Base#log_error gives you the flexibility to handle errors however you like. This is especially useful if your Rails application is hosted on a machine to which you have limited access: you can have errors sent to you, instead of written to a file you might not be able to see. Or you might prefer to record the errors in a database, so that you can look for patterns.

The method ApplicationController#log_error is declared private to avoid confusion. If it weren't private, all of the controllers would think they had a log_error action defined. Users would be able to visit /<controller>/log_error and get Rails to act strangely.

See Also

  • Recipe 15.19, "Sending Mail with Rails"

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