Methods that deal directly with data and manipulation thereof, belong in the model, unless they are shared among models - in which case they belong in [concerns](http://api.rubyonrails.org/classes/ActiveSupport/Concern.html).

The `app/helpers` directory is generally reserved for view helpers. Also... there are some thoughts out there about not writing view helpers at all. See [this](http://nicksda.apotomo.de/2011/10/rails-misapprehensions-helpers-are-shit/). I tend to agree with that view.