There are different reasons why a ActiveRecord::StaleObjectError would occur. Sometimes it is caused because rails does not have a stable identity map yet. This means that if the same object would be retrieved via different associations, your rails process would not know they point to the same object, would keep different copies of the same object and that could easily cause a StaleObjectError if you attempt to change both.

Such an occurrence needs to be fixed structurally in your code. But sometimes you can get StaleObjectError because things happen at the same time. There are two ways to handle this:

• make sure that controller actions which affect the same object, are always executed sequentially. One way to achieve this is to lock the object you want to update in the database. Then all other processes wanting to update the same object will have to wait until the lock is released (and the lock can be acquired). This is a valid approach but costly, intrusive (you need to explicitly add code), and possibly dangerous. It is normally easy to avoid, but you have to be careful for deadlocks and locks that are never released.
• when a StaleObjectError occurs, just retry the request

Now that second option seems valid, if only it would be easy to handle. Luckily there is a very easy and unobtrusive way to automatically retry all requests when a StaleObjectError occurs. Let's just create a middleware that catches the exception and retries the complete request a few times.

Create a new file lib/middleware/handle_stale_object_error.rb containing the following:

module Middleware 
  class HandleStaleObjectError 
    RETRY_LIMIT = 3 
    
    def initialize(app) 
      @app = app 
    end 

    def call(env) 
      retries = 0 
      begin 
        @app.call(env) 
      rescue ActiveRecord::StaleObjectError 
        raise if retries >= RETRY_LIMIT 
        retries += 1 
        Rails.logger.warn("HandleStaleObjectError::automatically retrying after StaleObjectError (attempt = #{retries})") 
        retry 
      end 
    end 
  end 
end

Then in your config/application.rb you need to add the following lines at the bottom to activate the middleware (this requires a restart of your rails server):

 #
 # Middleware configuration
 #
 require 'middleware/handle_stale_object_error'
 config.middleware.insert_after ActiveRecord::SessionStore, Middleware::HandleStaleObjectError

Warning : this code will retry all occurences of StaleObjectError. For some occurrences this will not help at all, so you have to use your own judgement if this middleware is something you want in your codebase. I like this approach because it is an "optimistic" approach: it only adds extra processing when a StaleObjectError occurs, and when a not-fixable StaleObjectError it still fails as it should.

Hope this helps.

In some part of my code I ended writing the following:

self.count_processed ||= 0 
self.count_processed += 1

where self is some ActiveRecord model, and count_processed is an attribute of that model (and stored in the database). What am i trying to achieve (if it is not blatantly obvious):

• if count_processed is not initialised, make it zero
• increment count_processed

Imho this code is clear and readable, but I had the feeling it could be more concise/prettier. So I asked that question on our campfire, to see if we could come up with something shorter. Very nice to be working in a team where you can just throw up questions like these and a very useful, educational discussion unfolds. In short we came up with the following solutions.

Solution 1: to_i

self.count_processed = self.count_processed.to_i + 1

Nifty! Isn't it? Use to_i because it will handle the nil correctly. But for me this looked wrong. If I would return to this code after a few weeks, months, I would wonder why I did this way, and not just wrote self.count_processed += 1. So while the code is correct, the intent of the code is not clear.

Solution 2: concise!

self.count_processed = (self.count_processed || 0) + 1 

This is very beautiful, and the intent is also very clear. If it is not initialised, use the zero, else just use the value and add 1. Awesome.

Solution 3: change the getter

An alternative solution would be to overwrite the getter, like this

def count_processed 
  self[:count_processed] ||= 0 
end

Note the notation we used: we use self[:count_processed] because this will fetch the value from the database column. If this was a normal getter, we would write @count_processed (but that does not work for an ActiveRecord model). After redefining the getter, we can just write: self.count_processed += 1 While this will work always, does it express its intent more clearly or not? Actually you no longer have to worry about the initialisation, because it is handled, and we can focus on what we really want: increment the counter. I opted for this solution.

What about you?

Which version do you prefer? Do you have any alternative suggestions?

Since rails 3.2.4 the link_to_function is effectively deprecated (again). Everything keeps on working, but when running my specs I got a horseload of deprecation warnings. I know the optimal/recommended way is to use unobtrusive javascript, but the quickest way to fix this is really easy. Just perform the following translation:

    # before 
    link_to_function icon_tag('close.png'), '$(this).parent().hide()', :title => t('actions.close'), :class => 'close' 

    # after 
    link_to icon_tag('close.png'), '#', :onclick => '$(this).parent().hide()', :title => t('actions.close'), :class => 'close' 

Dead-easy :) In the next refactoring I will remove all onclick blocks and replace them with unobtrusive javascript. But for now I got rid of the deprecation warnings :)

I had an issue where I was not sure the ActiveRecord::SessionStore was actually working (in hindsight: it worked). But to make sure, I needed to know what was stored in the session or retrieved.

All logging for the session-store is silenced, using Base.silence.

Obviously I was very interested in that logging, and did not find another to unsilence the logging but to add an initializer with the following code.

So in file config/initializers/unsilence_logging.rb write:

class ActiveRecord::Base 
  def self.silence 
    yield self 
  end 
end

This will unsilence the SessionStore logging. Your logging will look like this:

 ^[[1m^[[36mAREL (0.0ms)^[[0m ^[[1mUPDATE "sessions" SET "data" = 'BAh7DEkiFnF1aWN<<snipped to protect the inncocent>>iEi9mcC9kYXNoYm9hcmQ= ', "updated_at" = '2012-05-04 11:17:24.704491' WHERE "sessions"."id" = 33635

This at least allows us to verify that the sessions are stored and retrieved correctly. But how can we see what is stored inside the session? To be able to read or inspect what is actually stored in the session, you can use the following line:

session_data = 'BAh7DEkiFnF1aWN<<snipped to protect the inncocent>>iEi9mcC9kYXNoYm9hcmQ= ' 
Marshal.load(ActiveSupport::Base64.decode64(session_data))

And this will present your session data in a readable format.

This way I learned that a time-drift between our two servers caused a very obscure bug. I hope it can help you too.

Assume you have, like I did, a ruby gem that contains some javascript you want to test standalone. First you need to install the jasmine-gem. You have two options:

• either you use your gemspec to drive your bundler gemfile, so just add it to your developement dependencies
• I am still using jeweler, so I use a normal Gemfile, which jeweler parses to populate my gemspec with. Personally I find this much easier, and my workflow is much closer to any ruby development for me, this way

If you have a rails-project, starting with jasmine is easy, and takes three easy steps:

# first add jasmine to Gemfile, and then 
bundle install 
rails g jasmine:install 
rake jasmine 

Inside your gem or simple ruby-project it is equally simple, just type

jasmine init 
rake jasmine

Now you can need to edit the jasmine.yml to make sure it is running your tests and your code, instead of the example code. In my case I had to change only one line:

src_files: - app/assets/javascripts/\*\*/\*.js 

Happy testing :) Some interesting links to help you with jasmine:

• a very interesting slideshow
• jasmine homepage

I really like generative art, so I have been playing with processing for a while. Processing is an open source language (on top of java), that gives the possibilty to create images, animations, with added interactivity. First I created a simulation of raindrops, and because I wanted it to be easily configurable I used processing.js: processing implemented on top of javascript. So that becomes native processing in the browser. Allowing to interact with HTML and javascript objects easily. Publishing the sketch is just: give people the link.

Next-up I wanted to create a music visualisation. Interact with the music. I had the perfect piece of music in my head: "Endless Season" by Ken Ishii. Now processing.js does not interact with music. There is HTML5 audio, but it still is very experimental, and I did not find any API for processing music, reacting to and analysing music played. Processing (the java version) has an excellent library for this: Minim (actually more than one, but I ended up using that one). When the sketch was finished, I wanted to share it, convert it to a video. What were the options.

Use Processing itself

There are two ways to convert a processing sketch to a movie, from within processing itself:

• use MovieMaker: this requires quicktime and unfortunately does not work on Ubuntu
• when each frame is drawn, do saveFrame and then convert all frames to a movie afterwards. While in theory this should work, saving the images slowed down my sketch, and ultimately screwed up the sync with the audio. My frame-rate was not consistent enough. For straightforward stuff this does not matter, but I needed it to sync with the audio.
• There is a third option: GSVideo, but frankly, that seemed to damn hard for me, so I skipped that. So I needed an alternative approach. If it runs correctly on my screen, couldn't I just record it on my screen?

Use some sort of screenrecording/screencast software

To record my desktop, on ubuntu, including the sound from processing proved to have some issues:

• processing (i.e. java/JDK 6) does not use ALSA to create the sound, but address the hardware devices directly
• I do not want to record my entire desktop, but a specific part, of a specific size
• I want to share my video on vimeo, so it has to follow certain guidelines The first proved to be the hardest.

Recording the system audio out together with the video

On ubuntu, I found one approach to work very well for me:

• use gtk-recordmydesktop
• use PulseAudio mixer, it will allow to take the sound output as input to record
• and record away! :) But, unfortunately, since java does not ALSA but uses the hardware devices directly, PulseAudio was unable to capture the sounds. However, by accident I found out that if you export your processing sketch to an applet, and run the applet in the browser, it does use ALSA and can be recorded perfectly. Awesome. Part one solved.

Recording a specific part of the screen

gtk-recordmydesktop allows to specify an area of the screen to record, but somewhat akwardly.

Now, for exporting to vimeo, it had to follow certain fixed, optimal sizes. E.g. 640×480 for 4:3 SD video, 640×360 for 16:9 SD video, and 1280×720 or 1920×1080 for HD. And that is hard to do if you are trying to position the recording box manually.

But, as I found out here, when using recordmydesktop from the commandline, you can send those options along:

  recordmydesktop -x=0 -y=0 --width=320 --height=240 

So, if you open the Advanced->Misc, and look for the Extra options field, there you can fill in the same options, and when you press record gtk-recordmydesktop will show the bounding box that is recorded.

Preparing your video for uploading to vimeo

To upload your video to vimeo, you have make sure two things are correct:

• the screensize, which we discussed before
• the video format

gtk-recordmydesktop creates an Ogg Theora file, and unfortunately vimeo does not accept that format yet. Converting that to MP4 was hell, until I found Arista Transcoder. Using Arista to create an MP4 is easy (but you have to know it):

• open Arista Transcode
• create a new conversion
• select your file, by default called out.ogv, as the source
• select Sony Playstation - PSP as the device
• press create!

This will create a file called out.mp4 which is just perfect for vimeo, including the sound.

The result

Twotoned from DIXIS on Vimeo.

I was investigating ways to generate pdf's in Ruby on Rails, but I had one enormous constraint: it had to deploy on heroku.

There are two very different ways to generate pdf's in ruby:

• use prawn: it is pure ruby, very powerful. It has it's own DSL, that unleaches all the power of building a PDF, but at the same time: it seems to be very hard and tedious.
• use some sort of HTML to PDF conversion. In ruby there exists two gems: wicked_pdf and PDFKit, both use wkhtmltopdf under the covers. I dreamed of having a view magically converted to PDF.

I went for the second option. Furthermore, I choose wicked_pdf over PDFKit, because I felt the rails integration was better. It allowed me to just render a view which would automatically be downloaded as a PDF.

Setting up wicked_pdf in Rails to run on heroku

Luckily, getting it running on heroku proved to be incredibly easy: just including the correct gem with the binaries that work on heroku.

In my Gemfile I added the following:

 gem "wicked_pdf"
 gem "wkhtmltopdf-heroku", :git => 'git://github.com/camdez/wkhtmltopdf-heroku.git' 

And, then, inside a view you want to render as pdf, write something like

 respond_to do |format|
   format.js 
   format.pdf { 
     render :pdf => "show", :header => { :font_size => '8', :right => '[page] of [toPage]' }, :footer => {:font_size => '8', :right => 'Generated by jottinx.com' } 
   } 
 end

Then, you will still have to create the view, show.pdf.erb. Just make sure your view renders HTML and it will be converted to PDF correctly. That is just awesome.

Hope this helps.

The Problem

For jottinx I wrote a small piece of code that allowed to sort items using drag and drop. Of course, after writing it and making sure it works (manually), I want to make sure it keeps working. So I add a test, using cucumber.

My scenario actually looks pretty straightforward:

 @javascript
 Scenario: Dragging a book
   Given I authenticate as a "default user"
   And I add a new book with title "a-new-book"
   And I add a new book with title "another-book"
   And I add a new book with title "one-last-book"
   And I drag book "one-last-book" to the top
   Then book "one-last-book" is at the top of the list

The difficult bit was: how do I implement the dragging?.

Actually it seemed straightforward, because capybara has a method called drag_to. So I implemented the step like this:

 When /^I drag book "([^"]\*)" to the top$/ do |book_title|
    drop_place = page.find(:css, 'ul.sortable-books li:first')
    page.find(:xpath, "//a[@href='##{book_title.parameterize}']").drag_to(drop_place)
 end

But, unfortunately, this did not work. I googled around a bit and found the following two similar questions:

• drag_to with jquery sortable list
• drag_to don't work on sortable elements

The short conclusion: it does not work, and it is a combination of how jquery implemented the sortable element, and the fact that the selenium driver does not support it yet. So, refraining to using the selenium driver directly does not help either. After some more googling, I found a similar question on stackoverflow, and there I found the solution.

The solution

Enter jquery.simulate.drag-sortable.js. It is a script that will allow you to simulate dragging in a sortable object by issuing a simple javascript command:

 // drag item down one position in the list
 $('#itemToDrag').simulateDragSortable({ move: 1 });

If move parameter is negative, it will move up. And down if positive. If you include the js inside your project, you can easily test that out inside your Chrome console. It just works. Awesome piece of work. To use that in a step-definition, just write:

 When /^I drag book "([^"]\*)" to the top$/ do |book_title|
    page.execute_script %Q{ 
      $('.sortable-books li:last').simulateDragSortable({move: -4}); 
    }
 end

Hope this helps :)