I can create a new book, as desired, in pair with its author by running: > book = Book.create(:title => "title", :author_attributes => {:name => "name"}, :text => "text") However, every attempt to post via http results in error: "Author can't be blank" I've already tried with: [...] <%= fields_for :author , @book.author do |ff| %> <li>Author<br>…

author.rb

class Author < ActiveRecord::Base
  validates :name,  :presence   => true
  has_many  :books, :inverse_of => :authors
end

book.rb

class Book < ActiveRecord::Base
  validates :title,     :presence => true
  validates :author,    :presence => true
  validates :text,      :presence => true

  before_create :fix_title
  belongs_to :author, :inverse_of => :books

  accepts_nested_attributes_for :author

  def fix_title
    self.title = self.title.titleize
  end
end

books_controller

class BooksController < ApplicationController

  def index
    @books = Book.all

    respond_to do |format|
      format.html
      format.yaml { render text:@books.to_yaml }
      format.json { render text:@books.to_json }
    end
  end

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

    respond_to do |format|
      format.html
      format.yaml { render text:@book.to_yaml }
    end
  end

  def new
    @book = Book.new
  end

  def create
    # @author = Author.create(params[:name])
    @book = Book.new(book_params)
    if @book.save
      redirect_to book_path(@book)
    else
      render :new
    end
  end

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

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

  private

  def book_params
    params.require(:book).permit(:title, :text, :author_id, :author_attributes)
  end
end

form.html.erb

<%= form_for(setup_book(@book)) do |f| %>
  <% if @book.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@book.errors.count, "error") %> prohibited this book from being saved:</h2>

      <ul>
      <% @book.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>
<ul>
  <li>Title <br>      <%= f.text_field :title   %></li>
  <%= fields_for :author_attributes , @book.author do |ff| %>
    <li>Author<br> <%= ff.text_field :name %>
  <% end %>
  <li>Text  <br>      <%= f.text_area  :text    %></li>
</ul>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>