Capistrano deploy assets precompile never compiles assets, why? - ruby-on-rails-3

Question

Using deploy.rb to precompile rails assets only when they change, this task is always skipping the compile of my assets :(
namespace :assets do
task :precompile, :roles => :web, :except => {:no_release => true} do
from = source.next_revision(current_revision)
if capture("cd #{latest_release} && #{source.local.log(from)} vendor/assets/ app/assets/ | wc -l").to_i > 0
run %Q{cd #{latest_release} && #{rake} RAILS_ENV=#{rails_env} #{asset_env} assets:precompile}
else
logger.info "Skipping asset pre-compilation because there were no asset changes"
end
end
end
What could causing this complete task not compiling? It always thinks there are no asset changes and throws that message.
I also never really understood the task, for example what does below source.log.local refer to?
source.local.log
Could anyone clarify what the task commands do and has some pointers why it never sees any asset changes? Thank you

Answer 1

What it does:
from = source.next_revision(current_revision)
source is a reference to your source code, as seen through your SCM (git, svn, whatever). This sets from as (essentially) the currently deployed version of your source code.
capture("cd #{latest_release} && #{source.local.log(from)} vendor/assets/ app/assets/ | wc -l")
capture means 'execute this command in the shell, and return its output'. The command in question references the log of changes to your source comparing the deployed version to the current version (specifying the paths where assets live as the ones that 'matter'), and passes that into the word count tool (wc -l). The -l option means that it returns a count of the number of lines in the output. Thus, the output (which is returned by capture) is the number of filenames that have changes between these two versions.
If that number is zero, then no files have changed in any of the specified paths, so we skip precompiling.
Why it doesn't work:
I don't know. There doesn't seem to be anything wrong with the code itself - it's the same snippet I use, more or less. Here's a couple of things that you can check:
Does Capistrano even know you're using the asset pipeline? Check your Capfile. If you don't have load 'deploy/assets' in there, deploying won't even consider compiling your assets.
Have you, in fact, enabled the asset pipeline? Check application.rb for config.assets.enabled = true
Do you have incorrect asset paths specified? The code is checking for changes in vendor/assets/ and app/assets/. If your assets live somewhere else (lib/assets, for instance), they won't be noticed. (If this is the case, you can just change that line to include the correct paths.
Have you, in fact, changed any assets since the last deployment? I recommend bypassing the check for changed assets and forcing precompile to run once, then turning the check back on and seeing it the problem magically resolves itself. In my example, below, setting force_precompile = true would do that.
What I use:
Here's the version of this I currently use. It may be helpful. Or not. Changes from the original:
A more readable way to specify asset paths (if you use it, remember to set asset_locations to the places your assets live)
An easy way to force precompile to run (set force_precompile=true to attempt to run the check, but run precompile regardless)
It prints out the count whether or not precompile runs. I appreciate getting some output just to be sure the check is running.
If an error occurs when trying to compare the files (as will often happen with brand new projects, for instance), it prints the error but runs precompile anyway.
.
namespace :assets do
task :precompile, :roles => :web, :except => { :no_release => true } do
# Check if assets have changed. If not, don't run the precompile task - it takes a long time.
force_compile = false
changed_asset_count = 0
begin
from = source.next_revision(current_revision)
asset_locations = 'app/assets/ lib/assets vendor/assets'
changed_asset_count = capture("cd #{latest_release} && #{source.local.log(from)} #{asset_locations} | wc -l").to_i
rescue Exception => e
logger.info "Error: #{e}, forcing precompile"
force_compile = true
end
if changed_asset_count > 0 || force_compile
logger.info "#{changed_asset_count} assets have changed. Pre-compiling"
run %Q{cd #{latest_release} && #{rake} RAILS_ENV=#{rails_env} #{asset_env} assets:precompile}
else
logger.info "#{changed_asset_count} assets have changed. Skipping asset pre-compilation"
end
end
end