If you have a Rails application that needs to include models from another Rails application, oftentimes you will want to add features to those methods, but you cannot since you cannot touch the other Rails app's codebase. If this is the case, you can use class_eval, which Neeraj explains nicely. For example, if you want to add an after_save to a class Example :

Example.class_eval do after_save :somemethod def somemethod end end

Note that you dont wanna put that code under vendor/plugins/ . Instead, put it under a place where it's executed everytime.