# Terminal
rails plugin new books_module --mountable

# test/dummy/config/routes.rb
Rails.application.routes.draw do
  resources :users
  mount BooksModule::Engine => "/books_module", as: :books_engine
  root to: 'users#index'
end

# Terminal
cd test/dummy
rails g scaffold users first_name last_name
rake db:migrate

cd ../..
rails g scaffold books name year:date
rake db:migrate

rails g migration add_book_id_to_users book_id:integer:index
rake db:migrate

rake books_module:install:migrations

# test/dummy/app/views/users/index.html.erb
<%= link_to 'Books', books_engine.books_path %>

# app/views/books/index.html
<%= link_to 'Users', main_app.users_path %>

# lib/books_module.rb
require "books_module/engine"

module BooksModule
  require_relative 'books_module/model.rb'
end

# lib/books_module/model.rb
require_relative 'models/user.rb'

# lib/books_module/models/user.rb
module BooksModule
  module UserModel
    extend ActiveSupport::Concern
    included do
      belongs_to :book, class_name: 'BooksModule::Book', optional: true

      def has_favorite_book?
        book.present?
      end
    end
  end
end

# lib/test/dummy/app/models/user.rb
class User < ApplicationRecord
  # belongs_to :book, class_name: 'BooksModule::Book', optional: true

  # def has_favorite_book?
  #   book.present?
  # end
  include BooksModule::UserModel
end

# app/models/books_module/book.rb
module BooksModule
  class Book < ApplicationRecord
    has_many :users
  end
end

# test/dummy/app/views/users/_form.html.erb
    <%= form.label :book_id %>
    <%= form.collection_select :book_id, BooksModule::Book.all, :id, :name, include_blank: true %>

# test/dummy/app/controllers/uesrs_controller.rb
    private
    ...
    def user_params
      params.require(:user).permit(:first_name, :last_name, :book_id)
    end