This post originated from an RSS feed registered with Ruby Buzz by rwdaigle. Original Post: What's New in Edge Rails: Nested Model Mass Assignment Feed Title: Ryan's Scraps Feed URL: http://feeds.feedburner.com/RyansScraps Feed Description: Ryan Daigle's various technically inclined rants along w/ the "What's new in Edge Rails" series. Latest Ruby Buzz Posts Latest Ruby Buzz Posts by rwdaigle Latest Posts From Ryan's Scraps

Nested models (nested forms by another name) describe the scenario when you want to create and modify values of nested attributes through a containing model. For instance, if you have an user model with many phone numbers:

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class User < ActiveRecord::Base
  validates_presence_of :login
  has_many :phone_numbers
end

class PhoneNumber < ActiveRecord::Base
  validates_presence_of :area_code, :number
  belongs_to :user
end

you may want to be able to create the user and a group of phone numbers at the same time. This is what this looks like with the new mass assignment functionality of Rails keyed off of the :accessible option of the association declaration (:phone_numbers, in this case).

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class User < ActiveRecord::Base
  validates_presence_of :login
  has_many :phone_numbers, :accessible => true
end

ryan = User.create( {
  :login => 'ryan',
  :phone_numbers => [
    { :area_code => '919', :number => '123-4567' },
    { :area_code => '920', :number => '123-8901' }
  ]
})

ryan.phone_numbers.count #=> 2

A single hash of values being sent to User.create results in both a new user object and new associated phone numbers. Previously, you would have had to manually create your own phone_numbers= setter method on user to get this same functionality:

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class User < ActiveRecord::Base

  ...

  def phone_numbers=(attrs_array)
    attrs_array.each do |attrs|
      phone_numbers.create(attrs)
    end
  end

end

Mass assignment now gives you this functionality for free.

This may not look like much, but it is a step in the direction of letting you use nested forms. Consider a user registration form where a user can enter their login name and their phone numbers in the same form (through the use of fields_for which will bundle nested model attributes into a single form):

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<% form_for @user do |f| %>
  <%= f.text_field :login %>
  <% fields_for :phone_numbers do |pn_f| %>
    <%= pn_f.text_field :area_code %>
    <%= pn_f.text_field :number %>
  <% end %>
  <%= submit_tag %>
<% end %>

This form, when submitted to the following standard RESTful UserController, will correctly create the user and its associated phone numbers through the beauty of mass assignment.

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class UserController < ApplicationController

  # Create a new user and their phone numbers with mass assignment
  def new
    @user = User.create(params[:user])
    respond_to do |wants|
      ...
    end
  end
end

Mass assignment can be used on all association types – :belongs_to, :has_one, :has_many and :has_and_belongs_to_many as long as the :accessible => true option is specified.

This is a very convenient addition to ActiveRecord, but the real zinger will come with full nested form support when you can create, update and delete these nested models directly from what is pushed down in the parameter hash of a form submission. This would allow for the functionality in this complex forms screencast with minimal hassle. What a fine day that would be.

tags: ruby, rubyonrails