has_many: через ассоциацию и form_for

Question

Я пытаюсь впервые построить форму, включая has_many через ассоциацию это мои три модели:

class Book < ActiveRecord::Base 
has_many :talks 
has_many :authors, through: :talks 
accepts_nested_attributes_for :talks 
accepts_nested_attributes_for :authors 
validates :title, presence: true 

end 

class Author < ActiveRecord::Base 
has_many :talks 
has_many :books, through: :talks 
validates :l_name ,presence: true 
end 

class Talk < ActiveRecord::Base 
belongs_to :book 
belongs_to :author 
accepts_nested_attributes_for :author 
end 

моя книга/new.html.erb

<h1>Add new book</h1> 
<%= form_for (@book) do |f| %> 
    <div> 
    <%= f.label :title %><br> 
    <%= f.text_field :title %> 
    </div> 
    <div> 
    <%= f.fields_for :talks, Talk.new do |t| %> 
     <div> 
     <%t.fields_for :author, Author.new do |a| %> 
     <%= a.label :f_name %> 
     <%= a.collection_select :f_name, @authors, :f_name, :f_name %> 
     <%end %> 
     </div> 
    <% end%> 
    </div> 
    <div> 
    <%= f.submit %> 
    </div> 
<%end%> 

мой books_controller.rb

def new 
@book =Book.new 

end 
def create 
@book= Book.new(book_params) 
if @book.save 
    flash[:notice]='goood' 
    redirect_to admin_manage_path 
else 
    flash[:alert]='ouups' 
    redirect_to root_url 
end 
private 

def book_params 
params.require(:book).permit(:title, :pages, :resume, authors_attributes: [talks_attributes: []]) 
end 
end 

Я получил эту ошибку:

undefined method `map' for nil:NilClass

и я пытался следовать этому посту: http://howilearnedrails.wordpress.com/2013/12/18/rails-4-dropdown-menu-with-collection_select/

Answer 1

ли добавить ниже пропущен под 'новыми' действиями 'books_controller.rb'

@authors = Author.all