Ruby on rails : undefined method `map' for nil:NilClass , what causes this? when i add record to table

Question

here is my _form.html.erb

<%= simple_form_for @book,:html => { :multipart => true }  do |f| %>
<%= select_tag(:category_id, options_for_select(@categories), :promt => "Select a category") %>
<%= f.file_field :book_img %>
<%= f.input :title, label: "Book Title" %>
<%= f.input :description %>
<%= f.input :author %>
<%= f.button :submit %>

and here is my BooksController

class BooksController < ApplicationController

before_filter :initialize_book
before_action :find_book, only: [:show, :edit, :update, :destroy]

def initialize_book
    @book = Book.new
end


def show
    @book =Book.find(params[:id])
end

def index
    if params[:category].blank?
        @books = @books = Book.all.order("created_at DESC")
    else
        @category_id = Category.find_by(name: params[:category]).id
        @books = Book.where(:category_id => @category_id).order("created_at DESC")
    end     
end

def new
    @book = current_user.books.build  # Book.new 
    @categories = Category.all.map{ |c| [c.name, c.id]} 
end

def create
    @book =current_user.books.build(book_params)  # Book.new(book_params) 
    @book.category_id = params[:category_id]
    if @book.save
        redirect_to root_path
    else
        render 'new'
    end
end

def edit
    @categories = Category.all.map{ |c| [c.name, c.id]} 
end

def update
    @book.category_id = params[:category_id]
    if @book.update(book_params)
        redirect_to book_path(@book)
    else
        render 'edit'
    end
end

def destroy
    @book.destroy
    redirect_to root_path
end

private
    def book_params
        params.require(:book).permit(:title, :description, :author, :category_id, :book_img)
    end

    def find_book
        @book =Book.find(params[:id])
    end

end

book.rb

class Book < ActiveRecord::Base
	belongs_to :user
	belongs_to :category
	has_attached_file :book_img, styles: { book_index: "250x250>", book_show: "325x475>" }, default_url: "/images/:style/missing.png"
  	validates_attachment_content_type :book_img, content_type: /\Aimage\/.*\z/
end

I installing the paperclip gem to add image to Book and this is a file created when i run Rails migration generator: rails generate paperclip Book book_img

class AddAttachmentBookImgToBooks < ActiveRecord::Migration
  def self.up
    change_table :books do |t|
      t.attachment :book_img
    end
  end

  def self.down
    remove_attachment :books, :book_img
  end
end

But when i add Book and attrack an image to Book had show an error

NoMethodError in Books#create
Showing C:/Sites/BookReview/app/views/books/_form.html.erb where line #2 raised:

undefined method `map' for nil:NilClass
Trace of template inclusion: app/views/books/new.html.erb

Rails.root: C:/Sites/BookReview

Application Trace | Framework Trace | Full Trace
app/views/books/_form.html.erb:2:in `block in _app_views_books__form_html_erb___232429848_42489936'
app/views/books/_form.html.erb:1:in `_app_views_books__form_html_erb___232429848_42489936'
app/views/books/new.html.erb:3:in `_app_views_books_new_html_erb__358042162_78928608'
app/controllers/books_controller.rb:39:in `create'
Request

Parameters:

{"utf8"=>"✓",
 "authenticity_token"=>"kuaFBEmKqeVwLRr+NWke5AS5GxqPK0O/rOSPwDrc+8GsERzW0AlTJvhUji8/OevAIYhETyKc+jiNC3XIvpTAxQ==",
 "category_id"=>"3",
 "book"=>{"book_img"=>#<ActionDispatch::Http::UploadedFile:0x91a4060 @tempfile=#<Tempfile:C:/Users/muitr/AppData/Local/Temp/RackMultipart20160822-11652-1id5vn1.jpg>,
 @original_filename="csharp.jpg",
 @content_type="image/jpeg",

Answer 1

You set categories only for edit action, but what with new? Try this:

before_action :set_categories, only: [:new,:create, :edit]
private
def set_categories
@categories = Category.all.map{ |c| [c.name, c.id]} 
end

Answer 2

This line app/controllers/books_controller.rb:39:in create' says the error occours within method create. Within create you don't set @categories. Your before filter and action only set @book

render :new does not execute your new method. It only renders the given template for new. So @categories is still nil