House 9: Ajax error handling with ruby on rails and jquery

This sample is just a slight modification of this post 'Handling AJAX errors and displaying friendly error messages to users'. Basically I wanted to maintain the existing rails error handling for non-ajax pages and then have good handling for errors in both development and production mode; i.e. get errors in the browser when in development mode and show a nice message when in production mode.

Modify /app/controllers/application.rb; add the rescue_from macro and the handler_exception method

	class ApplicationController < ActionController::Base
	helper :all # include all helpers, all the time
	# ...
	rescue_from Exception, :with => :handler_exception
	# ...

	def handler_exception(exception)
	if request.xhr? then
	message = "Error: #{exception.class.to_s}"
	message += " in #{request.parameters['controller'].camelize}Controller" if request.parameters['controller']
	message += "##{request.parameters['action']}" if request.parameters['action']
	message += "\n#{exception.clean_message}"
	message += "\n\nFull Trace:\n#{exception.clean_backtrace.join("\n")}"
	message += "\n\nRequest:\n#{params.inspect.gsub(',', ",\n")}"
	# log the error
	logger.fatal "#{message}"
	message = "ajaxError, check the server logs for details" unless local_request?
	respond_to do \|wants\|
	wants.js { render :text => message, :status => :internal_server_error }
	end
	else
	# not an ajax request, use the default handling;
	# actionpack-2.2.2/lib/action_controller/rescue.rb
	rescue_action_without_handler(exception)
	end
	return # don't risk DoubleRenderError
	end
	end

view raw gistfile1.rb hosted with ❤ by GitHub

Then add a global javascript method for presenting the errors; most likely in application.js

	function defaultAjaxErrorHandler(result, isAjaxError) {
	// do stuff, hide spinner etc...
	var defaultAjaxError = "Your friendly error message";
	var errorToken = "Error:";

	if(!isUndefinedOrNull(result) && !isUndefinedOrNull(result.responseText)) {
	if(result.responseText.startsWith(errorToken)) {
	// localhost
	var x = (result.responseText.length > 350) ? 350 : result.responseText.length;
	alert(result.responseText.substring(0, x) + "...\n\n - Check firebug console for more info.\n - This message for localhost only.");
	}
	else {
	// production error
	alert(defaultAjaxError);
	}
	}
	else {
	// production error
	alert(defaultAjaxError);
	}
	}

	function isUndefinedOrNull(x) {
	var u; // undefined var
	if(x === u) { // similar to [typeof x == "undefined"]
	return true;
	}
	// else
	return x === null;
	}

view raw gistfile1.js hosted with ❤ by GitHub

Then a sample usage, jquery making an ajax post to a rails controller action

	$j.ajax({
	type: "POST",
	url: "/some_controller/some_action",
	data: {
	first_name: $j("#first_name").val(),
	last_name: $j("#last_name").val(),
	},
	success: function(data, textStatus) {
	// do what you like
	},
	error: function(result) { defaultAjaxErrorHandler(result, true); }
	});

view raw gistfile1.js hosted with ❤ by GitHub

in development mode the error message is returned in an easier to read view in the firebug console and an alert message is presented that has a truncated version of the error