book_author.rb

# == Schema Information
#
# Table name: book_authors
#
#  id          :bigint           not null, primary key
#  book_id     :bigint
#  author_name :string
#  created_at  :datetime         not null
#  updated_at  :datetime         not null
#
class BookAuthor < ApplicationRecord

    belongs_to :book

    validates :author_name, presence: true, uniqueness: { scope: :book_id,
        message: 'already added to this book'}
end