Form Objects and Transactions (multiple models)

Let's say we have two associated models:

# app/models/user.rb
class User < ApplicationRecord
  has_one :location
end

# app/models/location.rb
class Location < ApplicationRecord
  belongs_to :user
end

We want to create one instance of each model using a single registration form.
Ruby on Rails out of the box way is the _nested form_®™: fields_for, accepts_nested_attributes_for, maybe even inverse_of. This would require the following code at the least:

# app/views/registration/new.html.erb
<%= form_for @user do |f| %>
  <%= f.email_field :email %>

  <%= f.fields_for @user.build_location do |g| %>
    <%= g.text_field :country %>
  <% end %>
<% end%>

# app/models/user.rb
class User
  accepts_nested_attributes_for :location
end

Here’s what I already don’t like about this approach:

The view is coupled to the database structure. If we decide to make changes to the database schema later, the form will need to be updated.
Whitelisting attributes with strong parameters gets more complicated.
The User class contains logic to deal with Location’s attributes. This code is at odds with the Single Responsibility Principle. This is even more apparent when using reject_if.
It’s unclear what happens when save is called. If location is invalid, does user get saved? What if it’s the other way around?

So here’s an alternate proposal: use a form object!

class Registration
  include ActiveModel::Model

  attr_accessor :email, :password, :country, :city

  def save
    # Save User and Location here
  end
end

Meanwhile our view should look something like this:

<%= form_for @registration do |f| %>
  <%= f.label :email %>
  <%= f.email_field :email %>

  <%= f.input :password %>
  <%= f.text_field :password %>

  <%= f.input :country %>
  <%= f.text_field :country %>

  <%= f.input :city %>
  <%= f.text_field :city %>

  <%= f.button :submit, 'Create account' %>
<% end %>

And our controller like this:

class RegistrationsController < ApplicationController
  def create
    @registration = Registration.new(params)

    if @registration.save
      redirect_to root_url, notice: 'Registration successful!'
    else
      render :new
    end
  end
end

In our implementation we’ll return true from the save method if all models are saved and false if any of the models cannot be saved.

class Registration
  # ...

  def save
    return false if invalid?

    ActiveRecord::Base.transaction do
      user = User.create!(email: email, password: password)
      user.create_location!(country: country, city: city)
    end

    true
  rescue ActiveRecord::StatementInvalid => e
    # Handle exception that caused the transaction to fail
    # e.message and e.cause.message can be helpful
    errors.add(:base, e.message)

    false
  end
end

The trick here is to wrap the saving calls in a transaction and use create! instead of create. The transactions are rolled back when an exception is raised. This means that if one model fails to save then none of the models are saved. Finally, rescuing the error and returning false will signal that something went wrong.

Points worthy of note:

We can add an error not directly associated with an attribute by using the symbol :base:

validate :user_invite
def user_invite
  errors.add(:base, 'Missing invite token') unless token?
end

We can turn a database exception (like an email uniqueness constraint) into an error by doing something like:

rescue ActiveRecord::RecordNotUnique
  errors.add(:email, :taken)
end

For a more in-depth look at reusing database errors as validation errors, I suggest reading about uniqueness validations Show archive.org snapshot