My hovercraft is full of eels

This relatively unknown feature of subversion allows you to include one subversion repository URL inside another. By that I don't mean export and add, rather svn:externals allows you to create a virtual directory that links to an external repository.

I'm sure there are plenty of other uses but I mostly use svn:externals for adding plugins to my rails projects. For example, if you wanted to link to the Foreign Key Migrations plugin, from inside your rails project directory you would type:

simon$ svn propedit svn:externals vendor/plugins/

Then add the following lines:

foreign_key_migrations svn://rubyforge.org//var/svn/redhillonrails/trunk/vendor/plugins/foreign_key_migrations
schema_defining svn://rubyforge.org//var/svn/redhillonrails/trunk/vendor/plugins/schema_defining

And voila! Now when you update your project you'll see something like:

simon$ svn up 

Fetching external item into 'vendor/plugins/foreign_key_migrations'
External at revision 25.


Fetching external item into 'vendor/plugins/schema_defining'
External at revision 25.

At revision 2533.
simon$

Notice how subversion updates the virtual directories as well.

Be aware though that this does mean you'll always get the latest code when you update. This is exactly what I want but may not suitable on all projects. Sometimes a simple svn export is more appropriate.

We're currently toying with the possibility of using svn:externals to share common code between some projects that are presently all in one big code-soup project but will be split up over the next couple of months. I'll let you know what we decide to do in the end.