Worth pointing out here is that the stuff we put in app/models/concerns are concerns that apply to multiple classes. An example from HEY is eventable:
module Eventable
extend ActiveSupport::Concern
included do
has_many :events, as: :eventable, dependent: :destroy
...
class Access < ApplicationRecord
include Eventable
class Clip < ApplicationRecord
include Eventable
But the majority of our concerns are actually like those presented for the identity: Just a way of breaking up a large argument, like Identity, into smaller ones, like Accessor.
Now I understand that argument from people who say: well, Accessor should just be it’s own class! Yes, it could have been. You could do Accessor.new(identity).accessible_topics. But I find that style awkward. The fact that the identity is playing the role of an accessor is implicit in #accessible_topics.
So that’s another way of putting this: It’s a writing style. Like using subheads to explain subservient ideas within a broader context. You could probably extract all those subheads out, and turn them into little essays of their own. But that’s often just not the right level of extraction. As you saw, the Accessor role starts life as literally just two methods! It doesn’t warrant being turned into it’s own class. It doesn’t have an independent center of gravity.
As you’ll probably gather from all the writing metaphors, this isn’t a technical debate in the sense of “When x, then do y”. It’s a discussion of writing styles. Subtle breaking points. Careful evaluations.
But at the same time, you can absolutely teach style. A Rails Strunk & White would be great 
