Nested Model Forms

Bad Smell

class Product < ActiveRecord::Base
  has_one :detail
end

class Detail < ActiveRecord::Base
  belongs_to :product
end

<% form_for :product do |f| %>
  <%= f.text_field :title %>
  <% fields_for :detail do |detail| %>
    <%= detail.text_field :manufacturer %>
  <% end %>
<% end %>

class ProductsController < ApplicationController
  def create
    @product = Product.new(params[:product])
    @detail = Detail.new(params[:detail])

    Product.transaction do
      @product.save!
      @detail.product = @product
      @detail.save
    end
  end
end

Product and Detail models are one-to-one association, we want to create a product with a new manufacturer. In this example, we create a product and a detail object, and associate them in a transaction. But why we do the complex creation in the controller, Product model should take charge of creating a detail.

Refactor

class Product < ActiveRecord::Base
  has_one :detail
  accepts_nested_attributes_for :detail
end

<% form_for :product do |f| %>
  <%= f.text_field :title %>
  <% f.fields_for :detail do |detail| %>
    <%= detail.text_field :manufacturer %>
  <% end %>
<% end %>

class ProductsController < ApplicationController
  def create
    @product = Product.new(params[:product])
    @product.save
  end
end

We add the accepts_nested_attributes_for in Product model, so Product model can handle the creation of Detail model. That's so cool!

And we can also use accepts_nested_attributes_for to simplify the one-to-many association creation.

class Project < ActiveRecord::Base
  has_many :tasks
  accepts_nested_attributes_for :tasks
end

class Task < ActiveRecord::Base
  belongs_to :project
end

<% form_for @project do |f| %>
  <%= f.text_field :name %>
  <% f.fields_for :tasks do |tasks_form| %>
    <%= tasks_form.text_field :name %>
  <% end %>
<% end %>

Thanks rails for providing so awesome method!