I'm afraid you must look through the ActiveModel validation documentation / code: http://api.rubyonrails.org/classes/ActiveModel/Validations.html.

Maybe even the ActiveRecord implementation code help: http://api.rubyonrails.org/files/activerecord/lib/active_record/validations/uniqueness_rb.html

Overall this maybe will result in something where the `Collection` adds a validation to its associated `Model`.