Daniel Strunk

Cleaning up nested forms with form objects in Rails 4

It’s a big day today… Apple’s September 9th event, this blog’s 100th post, and perhaps most importantly, my dog Carl’s birthday! To celebrate, we’ll be talking about cleaning up nested forms in Rails with form objects.

Form Objects

As your application grows, your models might tend to get messy. Some people subscribe to the “fat model, skinny controller” viewpoint, but I think that our entire codebase should be easily digestible, and that means refactoring certain functionality out of a catch-all model and into plain ol’ ruby form objects, service objects et al. If you have a form that updates several ActiveRecord models with one submission, you’ve got a good candidate for a form object. Form objects can significantly clean up models that use accepts_nested_attributes_for ad nauseum. Before I get started, I should note that I’ll be dealing with Rails 4 for this example. In Rails 4, it’s easy to add ActiveRecord-like attribute functionality to objects by including include ActiveModel::Model in your class. That allows you to enjoy AR methods like validation, etc. One thing that tripped me up was getting strong parameters to work with this method. After learning how Rails expects its parameters however (hashes), everything started to Just Work&tm;. In this example, we’re going to deal with a form that updates both a User model that has one Profile model in a signup form. First, the models (basic stuff):

class User < AR::Base
	has_one :profile

	class Profile < AR::Base
	belongs_to :user
  • My User model has the following fields: :username and :email.
  • My Profile model has the following fields: :github and :twitter.

Now, instead of going the traditional Rails way with an accepts_nested_attributes_for :profile in our User model, we’ll extract out the entire form details into a separate signup object:

class Signup
	include ActiveModel::Model

	attr_accessor :username, :email, :twitter, :github

	validates :username, presence: true
	validates :email, presence: true, email: true
	validates :twitter, presence: true
	validates :github, presence: true

	def register
		if valid?


	def create_user
		hash = {}
		hash[:username] = "#{username}"
		hash[:email] = "#{email}"
		@user ||= User.new(hash)

	def create_profile
		hash = {}
		hash[:twitter] = "#{twitter}"
		hash[:github] = "#{github}"
		@profile = Profile.new(hash)
		@user.profile = @profile

Quick notes:

  • Include ActiveModel::Model to gain access to traditional AR methods. Quack, quack
  • attr_accessor for the fields we’ll be using
  • validations just like we’re used to
  • A register method that we’re using in our SignupsController (called below)
  • Our user and profile creation methods

Let’s take a look at our form. It’s standard stuff, but note that we’re using the object @signup given to us by the SignupsController (posted below the form):

<%= form_for(@signup) do |f| %>
	<% if @signup.errors.any? %>
	<div id="error_explanation">
	<h2><%= pluralize(@signup.errors.count, "error") %> prohibited this user from being saved:</h2>
	<ul><% @signup.errors.full_messages.each do |message| %>
	    <li><%= message %></li>
	<% end %>

	<% end %>
	<div class="field"><%= f.label :username %>

	<%= f.text_field :username %></div>
	<div class="field"><%= f.label :email %>

	<%= f.text_field :email %></div>
	<div class="field"><%= f.label :twitter %>

	<%= f.text_field :twitter %></div>
	<div class="field"><%= f.label :github %>

	<%= f.text_field :github %></div>
	<div class="actions"><%= f.submit %></div>
	<% end %>

From here it’s a simple matter of updating the controller accessing the form:

class SignupsController < ApplicationController
	respond_to :html

	def new
	@signup = Signup.new

	def create
	@signup = Signup.new(signup_params)
	respond_with @signup, location: some_location_path

	def signup_params
	params.require(:signup).permit(:user, :email, :github, :twitter)

This should look like something you’re used to. A simple create method passing in strong params, then calling the register method on the @signup object.

The end

That’s all for today! If you’d like to learn more about cleaning up your fat models, check out this great (if a bit dated) posts from codeclimate: 7 Ways to Decompose Fat ActiveRecord Models. It’s got great ideas that you should be able to update to work with Rails 4 pretty easily. I’ll close with a picture of Carl, hard at work building out a complex airbnb-type app for dogs: Carl, hard at work. Happy birthday Carl!