no such file to load — json?

While upgrading a project of mine to Rails 3 I stumbled upon a json error. I don’t know what exactly caused it since I have been upgrading from Ubuntu Karmic to Lucid in the past few days. Since this took a little head scratching I thought I would write it down here so I don’t forget. Hopefully it will save someone else some fooling around as well.
The error is coming from the jsvars plugin which is trying to call require ‘json’.

mike@sleepycat:~/projects/todasaulas$ rails s
=> Booting WEBrick
=> Rails 3.0.0 application starting in development on http://0.0.0.0:3000
=> Call with -d to detach
=> Ctrl-C to shutdown server
Exiting
/usr/lib/ruby/gems/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `require’: no such file to load — json (LoadError)
from /usr/lib/ruby/gems/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `require’
from /usr/lib/ruby/gems/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:225:in `load_dependency’
from /usr/lib/ruby/gems/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:591:in `new_constants_in’
from /usr/lib/ruby/gems/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:225:in `load_dependency’
from /usr/lib/ruby/gems/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `require’
from /home/mike/projects/todasaulas/vendor/plugins/jsvars/lib/jsvars.rb:1

It turns out that json is provided by libjson-ruby on Ubuntu so the fix was as easy as

mike@sleepycat:~$ sudo aptitude install libjson-ruby

I’m not sure why things were working before if that wasn’t there. Who knows all the stuff that gets changed in an upgrade. You learn something new everyday.