undefined method `model_name' for NilClass:Class BUT ist initialized - ruby-on-rails

Question

Hey i´ve got another problem ;)
When i try to create a new Book in my app it always says
undefined method "model_name" for NilClass:Class
I found out that it must be an uninitialized param in the Form_for function... Here my code:
NoMethodError in Books#new
Showing /app/views/books/_form.html.erb where line #1 raised:
undefined method `model_name' for NilClass:Class
Extracted source (around line #1):
1: <%= form_for(#book) do |f| %>
2: <% if #book.errors.any? %>
3: <div id="error_explanation">
4: <h2><%= pluralize(#book.errors.count, "error") %> prohibited this book from being saved:</h2>
Controller:
#GET /books/new
#GET /books/new.json
def new
#users = User.find(:all)
#book = Book.new
1.times{ #book.chapters.build }
#book.users = [current_user]
respond_to do |format|
format.html #new.html.erb
format.json { render json: #book }
end
end
I don´t know why it should be uninitialized, it worked properly before I changed some relations between Books and Users but I there shoudn´t be the failure or?
EDIT:
app/views/books/new.html.erb :
<h1>New book</h1>
<%= render 'form' %>
<%= link_to 'Back', books_path %>
And the Model :
class Book < ActiveRecord::Base
attr_accessible :abstract, :status, :titel, :user_tokens, user_ids, :chapters_attributes
has_and_belongs_to_many :users
attr_reader :user_tokens
has_many :chapters, :dependent => :destroy, :autosave => true, :order => 'slot'
validates :title, :presence => true
accepts_nested_attributes_for :chapters, :allow_destroy => true
after_initialize :init
def init
self.status = false if self.status?
end
def user_tokens=(ids)
self.user_ids = ids.split(",")
end
end
end

Answer 1

Your partial will not know the instance variables you set up in the controller. You will have to pass them as locals when you render the partial
render :partial => "form", :locals => {:book => #book}
And in your partial, use book instead of #book
<%= form_for(book) do |f| %>
<% if book.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(book.errors.count, "error") %> prohibited this book from being saved:</h2>