Handling Errors in Ajax & Rails

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A while ago, I was wondering how I should deal with errors that are encountered in rails controllers, while processing an Ajax request. I played around with some alternatives and eventually came up with one that has been working great for a few months, so I thought I'd share it.

You've probably all heard of overriding 'rescue_action_in_public' in your ApplicationController to deal with errors. (See simple example below).

def rescue_action_in_public(exception)
  # render the error page.
  render :file => "public/error.html"
end

This works great in regular methods, but what if something happens in an ajax request?

If you take no action, then the controller will fail 'silently' (well, some stuff appears in the log, but the user is unaware).

The way I got around this was to add a method to the ApplicationController, similar to that below (irrelevant code stripped out, so apologies if this code has a typo!).

  def perform_ajax_action
    
    if block_given?
      begin
        if (request.xhr?)
          yield
        else
          raise RuntimeError.new('Not in an ajax request!')
        end
      rescue Exception => error
        logger.error("#{Time.now}: Error performing ajax action: #{error.inspect}")
        logger.error(error.backtrace.join("\n"))
        ajax_aware_redirect_to "/error.html"
      end      
    else
      logger.error("perform_ajax_action called with no block!");
      ajax_aware_redirect_to "/error.html"
    end
    
  end

(You were probably wondering what this 'ajax_aware_redirect_to' business is all about. It's just another method in the ApplicationController which helps with redirecting during ajax processing. It looks a bit like this: Again, irrelevant stuff removed.)

  def ajax_aware_redirect_to(options = {}, *parameters_for_method_reference)
    if request.xhr?
      render :update do |page| 
        page.redirect_to( options, *parameters_for_method_reference) 
      end
    else
      redirect_to( options, *parameters_for_method_reference)
    end
  end

Now, in order to get this to catch your errors, you need to add an :around_filter to your controller. e.g.

class MyController < ApplicationController
  
  around_filter :my_around_filter, :only => [:my_action]
  
  # controller code here....

  def my_around_filter
    perform_ajax_action do
      # do other useful stuff here if you like.
      # in an 'around filter' yield calls the method wrapped in the filter
      yield
    end
  end
end