# ---role

module BookCreater
  def create_book(attrs)
    self.books.create! attrs
  end
end

module BookUpdater
  def update_book(book_id,attrs)
    book = self.books.find(book_id)
    book.update_attributes attrs
  end
end

module BookViewer
  def my_books
    self.books
  end
end

module BookDeleter
  def delete_book(id)
    self.books.find(id).destroy
  end
end

# ---context

class BookModifyingContext
  attr_accessor :user, :book
  def initialize(user_id)
    @user = User.find(user_id)
  end
end

class AddingBookContext < BookModifyingContext
  def initialize
    super
    @book = Book.new(:user => @user)
    @user.extend BookCreater
  end
  def execute(attrs)
    @user.create_book(attrs)
  end
end

class UpdateBookContext < BookModifyingContext
  def initialize
    @user.extend BookUpdater
  end

  def execute(id,attrs)
    @user.update_book(id,attrs)
  end
end

class DeletingBookContext < BookModifyingContext
  def initialize
    @user.extend BookDeleter
  end

  def execute(id)
    @user.delete_book(id)
  end
end

class ViewingBookContext < BookModifyingContext
  def initialize
    @user.extend BookViewer
  end
  
  def execute
    @user.my_books
  end
end

# ---controller

class BookController < ApplicationController
  before_filter :set_context, :only => [:new,:create]
  def index
    context = DeletingBookContext.new(params[:id])
    context.execute params[:book][:id]
  end
  def new
    @book = @adding_book_context.book
  end

  def create
    @adding_book_context.execute(params[:book])
  end
  
  def update
    context = UpdateBookContext.new(params[:id],params[:book])
    context.execute params[:book]
  end
  
  def destroy
    context = DeletingBookContext.new(params[:id])
    context.execute params[:book][:id]
  end
  
  private
    def create_context
      @adding_book_context = AddingBookContext.new(current_user.id)
    end
end