Rails validate_existence_of plugin

I'm reading the book Learning Rails by O'Reilly and it recommends using the validate_existence_of plugin. When I run:

ruby script/plugin install http: //svn.hasmanythrough.com/public/plugins/validates_existence/

it says it's already installed, but开发者_如何转开发 when I use it, I get a message saying:

NoMethodError in AwardsController#index 
undefined method `validates_existence_of' for #<Class:0xb5fde868>

When I say:

class Award < ActiveRecord::Base
  belongs_to :student
  validates_existence_of :student
end

Any ideas?

Have you checked that the plugin code is installed in the vendor/plugins directory ?
Also you may need to check that the environment.rb file has the config.gem section uncommented.