Question

I apologize if this may seem like a duplicate but I have not seen a solution explained clearly. I have a simple has_one, belongs_to association

class Author < ActiveRecord::Base
  attr_accessible :name, :book_attributes
  has_one :book
  accepts_nested_attributes_for :book, :allow_destroy => true  
end

class Book < ActiveRecord::Base
  attr_accessible :title, :author_id
  belongs_to :author
end

The authors_controller

class AuthorsController < ApplicationController
  def index
    @authors = Author.includes(:book).all

    respond_to do |format|
      format.html # index.html.erb
      format.json { render json: @authors }
    end
  end

 def show
    @author = Author.find(params[:id])

    respond_to do |format|
      format.html # show.html.erb
      format.json { render json: @author }
    end
  end

 def new
    @author = Author.new
    @book = @author.build_book    

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @author }
    end
  end

This Show.html.erb is the show stopper, the @author.book.title is giving me a undefined method for nil:NilClass:

<p id="notice"><%= notice %></p>

<p>
  <b>Name:</b>
  <%= @author.name %>
</p>

<p>
  <b>Book:</b>
  <%= @author.book.title %><br/>
</p>

<%= link_to 'Edit', edit_author_path(@author) %> |
<%= link_to 'Back', authors_path %>

Answer 1

It appears the Author you’re trying to show has a nil Book. So when you do @author.book.title, you’ll get the error since title is not a method on nil:NilClass.

To fix this, you’ll either need to check for a nil title by:

@author.book.try(:title)

Or just make sure all authors always have a book before they’re considered valid by adding this to your Author model:

validates :book_id, :presence => true