Ruby Rails - NoMethodError , drop-down list - ruby-on-rails

Question

I am trying to follow this tutorial. It has written in previous version of Rails and I am using Rails 4. There was a drop-down list the selection does not appear, I am getting following error:
NoMethodError in Book#show
Showing C:/Ruby193/mylibrary/app/views/book/show.html where line #3 raised:
undefined method `name' for nil:NilClass
Extracted source (around line #3):
1 <h1><%= #book.title %></h1>
2 <p><strong>Price: </strong> $<%= #book.price %><br />
3 <strong>Subject: </strong> <%= #subject.name %><br />
4 </p>
5 <p><%= #book.description %></p>
6 <hr />
Rails.root: C:/Ruby193/mylibrary
Here is the show.html
<h1><%= #book.title %></h1>
<p><strong>Price: </strong> $<%= #book.price %><br />
<strong>Subject: </strong> <%= #book.subject.name %><br />
</p>
<p><%= #book.description %></p>
<hr />
<%= link_to 'Back', {:action => 'list'} %>
Here is migrate/258412_subjects.rb
class Subjects < ActiveRecord::Migration
def self.up
create_table :subjects do |t|
t.column :name, :string
end
Subject.create :name => "Physics"
Subject.create :name => "Mathematics"
Subject.create :name => "Chemistry"
Subject.create :name => "Psychology"
Subject.create :name => "Geography"
end
def self.down
drop_table :subjects
end
end
Here is migrate/05465_books.rb
class Books < ActiveRecord::Migration
def self.up
create_table :books do |t|
t.column :title, :string, :limit => 32, :null => false
t.column :price, :float
t.column :subject_id, :integer
t.column :description, :text
end
end
def self.down
drop_table :books
end
end
Here is models/book.rb
class Book < ActiveRecord::Base
belongs_to :subject
validates_presence_of :title
validates_numericality_of :price, :message=>"Error Message"
end
Here is my controller class book_controller.rb
class BookController < ApplicationController
def list
#books = Book.all
end
def show
#book = Book.find(params[:id])
end
def new
#book = Book.new
#subjects = Subject.all
end
def create
#book = Book.new(book_params)
if #book.save!
redirect_to :action => 'list'
else
#subjects = Subject.all
render :action => 'new'
end
end
def edit
#book = Book.find(params[:id])
#subjects = Subject.all
end
def update
#book = Book.find(params[:id])
if #book.update_attributes(book_params)
redirect_to :action => 'show', :id => #book
else
#subjects = Subject.all
render :action => 'edit'
end
end
def delete
Book.find(params[:id]).destroy
redirect_to :action => 'list'
end
private
def book_params
params.permit(:title, :price, :description)
end
end
What should I do?, Thank you in advance

Answer 1

You should write
#book.subject.name
Of course then you should test if the subject is set. One way to do that is to write
#book.subject.try(:name)
This will will try name if subject is not nil.
And a general remark: do not seed the database from within a migration. Normally you will migrate only once, but when you go to production or on test, you will run rake db:setup which will create the database schema without running the migrations.
Instead use db/seed.rb to seed the database. This is then a repeatable process, which will also be needed when running the tests (which start from a clean/empty database).

Answer 2

According to your error
undefined method `name' for nil:NilClass
Extracted source (around line #3):
1 <h1><%= #book.title %></h1>
2 <p><strong>Price: </strong> $<%= #book.price %><br />
3 <strong>Subject: </strong> <%= #subject.name %><br />
#subject is nil in your show.
Since your controller has
def show
#book = Book.find(params[:id])
end
for your show, it seems likely that this is the case.
The code you have posted for your show, however, has the line as
<strong>Subject: </strong> <%= #book.subject.name %><br />
I would advise you to try restarting your server, as it seems to have cached an older version of your view.