I am not so sure. If you look at how dir structure is described for rails:

-   **app/models** - Holds models that should be named like post.rb. Models descend from
    ActiveRecord::Base by default.
-   **lib** - Application specific libraries. Basically, any kind of custom code that
    doesn't belong under controllers, models, or helpers. This directory is in
    the load path.

I see the module as a small app specific library for adding functionality to the app models, therefore I would place it in `lib` as `Validators` are placed.