Helper methods for models in Rails

Question

Is there a proper place for helper methods for models in Rails? There are helper methods for controllers and views, but I'm not sure where the best place to put model helper methods. Aside from adding a method to ActiveRecord::Base, which I'd prefer not to.

UPDATE: It seems Concerns make a lot of sense. Here's an example of what I want. Certain models can never be deleted, so I add a callback that always throws an exception:

before_destroy :nope
def nope
  raise 'Deleting not allowed'
end

With concerns, I could do something like this?

class MyModel < ActiveRecord::Base
  include Undeletable
end

module Undeletable
    extend ActiveSupport::Concern

    included do 
      before_destroy :nope 
    end

    def nope
      raise 'Deleting not allowed'
    end
end

Is this the Rails way of doing this?

Answer 1

If what you are asking is where to put code that is shared across multiple models in rails 4.2, then the standard answer has to be to use Concerns: How to use concerns in Rails 4

However, there are some good arguments (e.g. this) to just using standard rails module includes, and extends as marek-lipka suggests.

I would strongly recommend NOT using ApplicationController helper methods in a model, as you'll be importing a lot unnecessary baggage along with it. Doing so is usually a bad smell in my opinion, as it means you are not separating the MVC elements, and there is too much interdependency in your app.

If you need to modify a model object by adding a method that is just used within a view, then have a look at decorators. For example https://github.com/drapergem/draper