With the help of Jamie van Dyke at Parfait and Scott Chacon at GitHub, I'm pleased to announce Acts As Git (no, I don't like the name either). It's a simple plugin which stores all changes you make to a text field in a git repository. This is ideal for something like a git-backed wiki.

Look at it here: github or check it out from

git://github.com/courtenay/acts_like_git.git

From the README:

ALG automagically saves the history of a given text or string field. It sits over the top of
an ActiveRecord model; after a value is committed to the database, the plugin writes the
new value to a text file and commits it to a git repository. This way you get all the advantages
of using Git as version-control.

Usage:

class Post < ActiveRecord::Base
  versioning(:title) do |version|
    version.repository = '/home/git/repositories/postal.git'
    version.message = lambda { |post| "Committed by #{post.author.name}" }
  end
end

To view the complete list of changes:

>> @post = Post.find 15

>> @post.title
=> 'Freddy'
>> @post.history(:title)
=> ['Joe', 'Frank', 'Freddy]
>> @post.log
=> ['bfec2f69e270d2d02de4e8c7a4eb2bd0f132bdbb', '643deb45c12982dde75ba71657792a2dbdda83e6', 
'1ce6c7368219db7698f4acc3417e656510b4138d']
>> @post.revert_to '1ce6c7368219db7698f4acc3417e656510b4138d'
>> @post.title
=> 'Joe'

It uses the excellent Grit library, and doesn't actually have a checked-out repository. The latest version of your data is still stored in the database. You can actually clone this repo and view the changes; pushing back to it won't do anything useful.