Ruby-on-Rails

Error Checking and Debugging with Ruby on Rails

15.20 Automatically Sending Error Messages to Your Email

Problem

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

Solution

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

  private
    def log_error(exception)
       super
       Notification.deliver_error_message(exception,
         clean_backtrace(exception),
         session.instance_variable_get("@data"),
         params,
         request.env
       )
    end
  end

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    = _me@mydomain.com_
      @from          = _error@mydomain.com_
      @subject       = "Error message:
#{env[_REQUEST_URI_]}"
      @sent_on       = sent_on
      @body = {
        :exception => exception,
        :trace => trace,
        :session => session,
        :params => params,
        :env => env
      }
    end
  end

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_] %>

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

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

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

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

Discussion

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.