Snippets for Bookshelves

_form.html.erb

<%= form_for @bookshelf do |f| %>
  <%= f.error_messages %>
  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>

  <% @books.each do |book| %>
    <%= check_box_tag "books[#{book.id}]", "1", @bookshelf.books.include?(book)  %>
    <%= book.name %>
    <br>
  <% end -%>

  <p><%= f.submit %></p>
<% end %>

book.rb

class Book < ActiveRecord::Base
  has_many :chosen_books
  has_many :bookshelves, :through => :chosen_books
end

bookshelf.rb

class Bookshelf < ActiveRecord::Base
  has_many :chosen_books
  has_many :books, :through => :chosen_books
end

bookshelves_controller.rb

  # Note: inapplicable controller code omitted

  def update
    @bookshelf = Bookshelf.find(params[:id])
    if @bookshelf.update_attributes(params[:bookshelf])
      sync_selected_books
      redirect_to @bookshelf, :notice  => "Successfully updated bookshelf."
    else
      @books = Book.all
      render :action => 'edit'
    end
  end

  # ...

  private 

  def sync_selected_books
    if params[:books]
      checked_books = Book.where(:id => params[:books].keys).all
      current_books = @bookshelf.books

      books_to_remove = current_books - checked_books
      books_to_add = checked_books - current_books

      ChosenBook.delete_all(:bookshelf_id => @bookshelf.id, :book_id => books_to_remove)
      @bookshelf.books << books_to_add
    else
      ChosenBook.delete_all(:bookshelf_id => @bookshelf.id)
    end
  end

chosen_book.rb

class ChosenBook < ActiveRecord::Base
  belongs_to :book
  belongs_to :bookshelf
end