The simplest solution would be to create a module under lib and mix this into the models that need it, for instance, in lib/fooable.rb:
module Fooable
def do_foo
end
end
And then in your various models:
class MyModel < ActiveRecord::Base
include Fooable
end
No need to require fooable.rb, the Rails autoloading mechanism will find it for you as long as it’s named using correct conventions.