def revision_from_git
  if File.exists?(scope('.git/HEAD'))
   Dir.chdir scope(".") do
     `git rev-parse HEAD`
    end
  end
end

Since my git lives in a non-standard location, it was failing:

/Users/xxxxxxxx/.rvm/gems/ruby-1.9.2-p290@global/bundler/gems/compass-91a748a91636/lib/compass/version.rb:48:in ``': No such file or directory - git rev-parse HEAD (Errno::ENOENT)

Searching for a solution, I found a page in the EmacsWiki that dealt with this issue, but I didn’t care for any of the solutions there. What I went with was to open ~/.MacOSX/environment.plist with the Property List Editor, and add the path to my git executable to the PATH variable therein. I then had to reboot. The reboot seems to be the only way to get launchd to re-read this file. I launch Emacs via Spotlight, and so it is started by launchd behind the scenes. I tried using launchctl setenv first, as mentioned in the EmacsWiki, but that didn’t seem to work, I’m not sure why.

With the PATH variable set, rspec-mode, and rvm.el loaded, I was able to successfully run my specs from within Emacs.

(add-hook 'ruby-mode-hook
	  (lambda ()
	    (define-key ruby-mode-map "\C-c#" 'comment-or-uncomment-region)
	    )
	  )

This assigns C-c # to comment the current region, or if the current region is already commented, it will uncomment the region.

But what about when there’s no region currently marked? It would be nice if emacs would (un)comment the current line. To do this, I took a page from DJCB at Emacs-fu.

(defadvice comment-or-uncomment-region (before slick-comment activate compile)
  "When called interactively with no active region, comment a single line instead."
  (interactive
   (if mark-active (list (region-beginning) (region-end))
     (list (line-beginning-position)
	   (line-beginning-position 2)))))

Now with nothing marked, pressing C-c # will cause emacs to toggle commenting on the current line.

In case it isn’t obvious, one should add the abbove snippets to their .emacs file to gain their benefits.