accepts_nested_attributes_for - Rails association management using single form submit.

user.rb

# model file

class User < ActiveRecord::Base
  has_many :books
  accepts_nested_attributes_for :books, :reject_if => lambda {|book| book[:name].blank? }, :allow_destroy => true
end

# :reject_if => it passes the whole hash passed from browser with book details into this Proc(lambda) and iterates over each book hash. The book record is not created if the condition given in the lambda block is satisfied.

# :allow_destroy => if set to true, then the book record that was passed from browser, while submitting User form,  with a key-value pair :_destroy => 1 (or '1' or true or "true") will be destroyed

user_controller.rb

# controller file

class UsersControler < ActionController::Base

  def new
    @user = User.new
    @books = []
    @books << @user.books.new
  end

  def edit
    @user = User.find(params[:id])
    @books = @user.books
  end
end

user_form.html.haml

/ view file

= form_for @user do |form|
  = form.text_field :name

  = form.fields_for :books, @books do |book_name|
    = book_form.text_field :name
    = book_form.hidden_field :_destroy, :class => 'destroy-field'
    / this hidden_field should be given a value of ( 1 or "1" or true or 'true' ) if this record has to be destroyed.
      @books can either be @user.books if this is an User#edit form
      or can be @user = User.new; @user.build_books n times
      so that much times of form fields will be displayed of books belonging to user.