gistfile1.rb

class Shelf < ActiveRecord::Base
  has_many :comics
  has_many :books#, validate: -> {self.shelf_of == 'book'}

  accepts_nested_attributes_for :comics
  accepts_nested_attributes_for :books

  # has a dropdown on view for shelf_of that could either be 'comic' or 'book
  validates :shelf_of, inclusion: { within: %w(comic book) }
end

class Comic < ActiveRecord::Base
  
end

class Book < ActiveRecord::Base
  validates :title, format: { with: /\Asome_complicated_format\z/i }
end