Paul Ingles

The project I’m currently working on uses both the Asset Packager and Distributed Assets to ensure we have only a few external assets, and that we can load assets across more than one host - all so that the pages for our site load nice and quick.

Unfortunately, wiring in the Asset Packager plugin caused the Distributed Assets plugin to break, and I spent an hour or two tracking it down yesterday. The cause? Asset Packager redefines the compute_public_path method.

# rewrite compute_public_path to allow us to not include the query string timestamp
    # used by ActionView::Helpers::AssetTagHelper
    def compute_public_path(source, dir, ext=nil, add_asset_id=true)
      source = source.dup
      source << ".#{ext}" if File.extname(source).blank? && ext
      unless source =~ %r{^[-a-z]+://}
        source = "/#{dir}/#{source}" unless source[0] == ?/
        asset_id = rails_asset_id(source)
        source << '?' + asset_id if defined?(RAILS_ROOT) and add_asset_id and not asset_id.blank?
        source = "#{ActionController::Base.asset_host}#{@controller.request.relative_url_root}#{source}"
      end
      source
    end

Distributed Assets works by chaining compute_public_path - decorating the calculated path, adding the asset host prefix onto the url. But, Asset Packager works by defining the method into ActionView::Base. So, when DistributedAssets::AssetTagHelper is included with ActionView::Helpers::AssetTagHelper, it chains a (now) hidden method.

But, the only places that use the new compute_public_path code inside the Asset Packager Helper (which just avoids using the query string timestamp) is within Asset Packager itself.

So, I tweaked the implementation of AssetPackageHelper to

def compute_public_path_for_packager(source, dir, ext, add_asset_id=true)
  path = compute_public_path(source, dir, ext)
  return path if add_asset_id
  path.gsub(/\?\d+$/, '')
end
def javascript_path(source)
  compute_public_path_for_packager(source, 'javascripts', 'js', false)       
end

Beware the monkey patch.

I’d read a while back that Apple were going to include Sun’s DTrace tool in the now newly released Leopard - underpinning Instruments (formerly known as Xray).

What I hadn’t noticed was that the build of Ruby 1.8.6 included in Leopard (patchlevel 36 with some extras) also includes Ruby probes from the lovely folks at Joyent. Apple have ported everything. So, out of the box, Ruby works with DTrace! Sweet!

From Sun’s page:

DTrace is a comprehensive dynamic tracing facility that is built into Solaris and can be used by administrators and developers to examine the behavior of both user programs and of the operating system itself. With DTrace you can explore your system to understand how it works, track down performance problems across many layers of software, or locate the cause of aberrant behavior.

Joyent have got some sample scripts in their Subversion repository, and, what looks like some nice extensions for Rails (DTrace probes can be fired from within Ruby). Blimey!

For those (running Leopard) and want to just see something, try this out. It’s a script that traces the number of times a method is called. Once downloaded, all you need to do is

$ chmod +x functime.d
$ sudo ./functime.d -p <pid>

Replacing <pid> with the process id for your target Ruby process. Once you stop tracing (or your target process ends you’ll see some nice text formatted output. Including something like the following (from my current project):

Hash                     key?                     25123         15       384174
Module                   each_object                  1     390824       390824
Module                   included_in_classes          1     391025       391025
Module                   reloadable_classes_with      1     391203       391203
Module                   reloadable_classes           1     391835       391835
ActionController::Routin load_routes!                 1     394864       394864
ActionController::Routin reload                       1     398364       398364
Class                    reset_application!           1     398947       398947
Class                    reset_after_dispatch         1     400474       400474
ActionController::Routin connect                     39      13192       514506
Array                    include?                  3353        166       556605
ActionController::Routin build                       47      13116       616459
ActionController::Routin add_route                   47      13329       626488
Array                    each                       874        780       682423
Object                   load                         2     376432       752864
Module                   silence                      2     391586       783172
Kernel                   gem_original_require        28      43460      1216884
Array                    select                     316       4106      1297580
Array                    collect                    351       4124      1447524
Module                   local_constants             91      17120      1557986
Module                   new_constants_in            25      93885      2347139
Object                   require                     50      58495      2924783

The columns (from left to right) are: Class, Method, Count (number of times method was called) and then the average and total micro seconds. The d script was one of the Joyent examples. Arstechnica also have a very cool example showing the call stack from a system call.

What’s more amazing is this doesn’t require any recompilation, special flags, running in special environments. It’s available from the off!

If all that’s whetted your appetite (and I’m not sure how it couldn’t), head on over to the DTrace how to for a little more explanation of how it all works.

Time to get stuck in and see whether anything emerges about my current project. Very cool stuff!

In preparation for a Ruby project I wanted to play around with Rails’ page caching a little over the weekend. Naturally, I wanted to find a way to ensure that my controller was writing files correctly so that Apache (or any other HTTP server) can then serve the static content directly (without hitting my Rails processes). Since I’m also digging RSpec at the moment, I wanted to find a way of expressing it nicely in my specifications. This is what I ended up with.

Here’s the relevant controller’s specification:

describe UsersController, "caching" do
  include CachingExampleHelper
  it "should cache new users page" do
    requesting {get :new}.should be_cached
  end
  it "should not cache the activation page" do
    requesting {get :activate, :activation_code => 'blah'}.should_not be_cached
  end
end

The key part is the requesting line. That’s my extension that encapsulates the request that should be cached, so that we can both make the request and check that it ended up with a file being written to the filesystem.

module CachingExampleHelper
  ActionController::Base.public_class_method :page_cache_path
  ActionController::Base.perform_caching = true
  def requesting(&request)
    url = request.call.request.path
    ActionController::Base.expire_page(url)
    request.call
  end
  module ResponseHelper
    def cached?
      File.exists? ActionController::Base.page_cache_path(request.path)
    end
  end
  ActionController::TestResponse.send(:include, ResponseHelper)
end

Note that we have to expose the pagecachepath method so we can calculate the location for where the cached files will get written, and then also explicitly enable caching. When we call requesting giving a block to the request we want to make (such as get :index to invoke the index action), we capture the block as a Proc, invoke it and then read the request’s path from the generated TestResponse object. This lets us then first expire the page (in case it was cached previously), and then make the request. Finally, we then check whether the cached file exists by mixing in a cached? method to the Response.

Unfortunately, the code above will make 2 requests per assertion. But, I’d rather have it read nicely for the time being. Any suggestions on how you could tidy this up further would be well received!

I’m not sure I’d want to run these tests all the time, maybe something a little lighter to ensure that we tell Rails to caches_page for relevant actions in our controllers, but as a definite re-assurance it seems ok.

Inside my tests I try and keep everything I need inside the test, rather than depending on too many fixtures. That way I ensure they’re as readable as possible, and more importantly, obvious.

But, take the following example:

def test_total_is_due_now_if_return_is_less_than_14_days
  booking = Booking.new(
    :customer => customers(:valerie),
    :collect_at => 1.day.from_now.to_date,
    :return_at => 13.days.from_now.to_date,
    :boxes => 2,
    :collection_address => addresses(:one))
  assert_equal Date.today.to_s, booking.final_payment_due_date.to_s
end
def test_payment_due_14_days_from_now
  booking = Booking.new(
    :customer => customers(:valerie),
    :collect_at => 1.day.from_now.to_date,
    :return_at => 20.days.from_now.to_date,
    :boxes => 2,
    :collection_address => addresses(:one))
  assert_equal 6.days.from_now.to_date.to_s, booking.final_payment_due_date.to_s
end

Well, everything’s in the test, but there’s a fair bit of duplication. What would be nicer, is to take the prototype booking above and just override certain values.

Instead we might write something like

def test_total_is_due_now_if_return_is_less_than_14_days
  booking = new_booking_with(:return_at => 13.days.from_now.to_date)
  assert_equal Date.today.to_s, booking.final_payment_due_date.to_s
end
def test_payment_due_14_days_from_now
  booking = new_booking_with(:return_at => 20.days.from_now.to_date)
  assert_equal 6.days.from_now.to_date.to_s, booking.final_payment_due_date.to_s
end
private
def new_booking_with(args)
  Booking.new(args.reverse_merge({
    :customer => customers(:valerie),
    :collect_at => 1.day.from_now.to_date,
    :return_at => 15.days.from_now.to_date,
    :boxes => 2,
    :collection_address => addresses(:one)
  }))
end

Much nicer (to me anyway), particularly once you start adding more and more tests (and especially validation related ones).

I’m working on a little pet Rails project and wanted to use Mocha to isolate my controller tests a little from what is usually encouraged, especially in light of what happened last time. I figured I’d give Mocha a go.

I wanted to verify that my service was called with objects that look equal to what I was expecting. Ordinarily, it will compare instances - are they the same object. I didn’t want that, but rather wanted to test that the values of my (ActiveRecord) objects were the equal. Easy. Mocha allows with to be called with a block that is evaluated to compare the objects.

My tests look something like this

def test_should_ask_service_to_calculate_new_prices_for_booking
  address = Address.new(:postcode => "W1 1QE")
  booking = create_booking
  OrderBookingService.expects(:calculate_price).with(booking, address) do |b, a|
    b.attributes == booking.attributes && a.attributes == address.attributes
  end
  post :calculate_price, :booking => booking, :collection_address => address
end

It’s something I’ve had to do many times in the past with JMock and NMock - I want to test equality for an aspect of some other object but it’s always meant writing rather a lot of code. Neat.