Ruby On Rails has a Subversion repository with over 7500 commits at the time of writing. In this article I show how you can use Git to have a full local copy of the repository in 9MB, and use it to track upstream changes.

In a subsequent article I will show you how to use such a repository to easily maintain Rails patches for the six months it takes you to get them accepted into Core (wink).

Background

Git is a revision control system that is distributed, which means that every developer using Git gets to be the ruling dictator of his own distinct code repository. With a distributed RCS, a central “master” repository is an optional result of a consensus decision, and not a technical requirement.

Having a full local repository lets you work off-line, branch and merge freely, and check in regularly without being a project member.

Mirroring subversion repositories: git-svn

Git ships with a nifty tool called git-svn, which can pull and push changesets between a subversion repository and a git repository, maintaining tag, branch and author information.

After reading a helpful article I used git-svn to make a mirror of the Rails subversion repository. That step took many hours, so I’ve set up my mirror to update itself regularly throughout the day, and published it at the following Git URL:

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git://git.sanityinc.com/rails.git

More on how to use that URL later.

What you will need

Make sure you have git installed, with svn support. (To follow this article, you don’t need the svn support, but you’ll no doubt want to play with git-svn yourself.) In most Linuxes, the package to install is called “git-core”. On a Mac, if you value your time you will use the MacPorts package with ‘svn’ variant enabled:

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% sudo port install git-core +svn +doc

Make a local clone of the repository

You can clone my Git mirror of the Rails repository using git-clone:

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% git-clone git://git.sanityinc.com/rails.git
Initialized empty Git repository in /tmp/rails/.git/
remote: Generating pack...
remote: Counting objects: 6041
Done counting 59416 objects.
remote: Deltifying 59416 objects...
 100% (59416/59416) done16) done
Indexing 59416 objects...
remote: Total 59416 (delta 45829), reused 58267 (delta 45039)
 100% (59416/59416) done
Resolving 45829 deltas...
 100% (45829/45829) done

The newly-created rails directory contains every changeset in Rails’ history, all stored in only 9.2MB of packed Git data:

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% cd rails % du -hs .git/objects
9.2M    .git/objects

This magic is possible due to Git’s delta compression. Note that a plain vendor/rails directory alone weighs in at over 8MB.

Here are all the branches:

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% git-branch -a
* master
  origin/1-1-stable
  origin/1-2-pre-release
  origin/1-2-stable
  origin/HEAD
  origin/cap2-on-netssh2
  origin/cap2-on-netssh2@2073
  origin/cap2-on-netssh2@3784
  origin/cap2-on-netssh2@7243
  origin/capistrano_1-x-stable
  origin/capistrano_1-x-stable@2073
  origin/capistrano_1-x-stable@3784
  origin/capistrano_1-x-stable@6222
  origin/master
  origin/p2
  origin/performance
  origin/restful_aws
  origin/restful_aws@1324
  origin/restful_aws@668
  origin/routing
  origin/stable
  origin/trunk

Note that the default branch is master, which corresponds to origin/trunk - Rails “edge”.

You can trivially switch the working copy in the current directory to another branch, such as the Rails stable branch:

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% git-checkout -b stable origin/stable
Branch stable set up to track remote branch refs/remotes/origin/stable.
Switched to a new branch "stable"
</blockcode>
(It's considered bad practice to check out remote branches directly, so here we made a local 'stable' branch that tracks 'origin/stable'.)

'git-log' gives us the commit history for the current branch:
<blockcode>
% git-log
commit 1f5a5285c45caf49261ded485aa9994296e0305e
Author: ulysses <ulysses@5ecf4fe2-1ee6-0310-87b1-e25e094e27de>
Date:   Thu Aug 10 17:13:55 2006 +0000

    Can't use controller_path due to Admin model and Admin::UserController case

    git-svn-id: http://svn.rubyonrails.org/rails/branches/stable@4750 5ecf4fe2-1ee6-0310-87b1-e25e094e27de
...

Note the embedded subversion revision, @4750.

You can browse the history of the current branch using the gitk GUI.

Keeping your mirror up to date

You’ll want to periodically pull new changes from the Rails repository into your local repository. Use ‘git-pull’ for this:

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% git-pull
remote: Generating pack...
remote: Done counting 23 objects.
remote: Result has 15 objects.
remote: Deltifying 15 objects...
remote:  100% (15/15) done
remote: Total 15 (delta 13), reused 0 (delta 0)
Unpacking 15 objects...
 100% (15/15) done
* refs/remotes/origin/trunk: fast forward to branch 'trunk' of git://git.sanityinc.com/rails
  old..new: 258ae9d..6bf2bef
Already up-to-date.

Making changes locally

With a little git knowledge, you can commit changes into your local repository, getting git to do the magic of merging upstream changes.

If you want to contribute patches back to Rails, you can use git to maintain them; git can regenerate your patches automatically as the surrounding Rails code changes. A follow-up article on this site will describe how to do this.

Further reading

• Git documentation
• This site – why not subscribe to future articles?

Update: I’ve set up gitweb, so you can browse the Rails git mirror here.

Further update: my mirror will be discontinued now that Rails development has switched to Git