In previous tutorial we discussed refactoring code and decorators pattern. In this tutorial we will take our discussion further to form objects pattern. We use forms in our application to take inputs from user. The form data is received in our controller, where we have some logic to check data and save inputs to database. While refactoring our code we should keep single responsibility principle (SRP) in mind. SRP states that a class should have single responsibility of a functionality.

A form object is nothing but the replacement of model object for form. The form object should respond to methods like active record objects.

Model objects generally have lots of functionality. All validations are placed in model that may be needed in certain cases only and hardly needed to be in model. We can extract these validations out from model using form objects. Please note, validations are important and should be in model to check the data integrity. We can not just extract all the validations out of model.

Form objects is also useful in the cases where we create multiple model objects in same form. Its better to use form objects than accepts_nested_attributes_for.

The form class will have all the attributes of the form fields. All we need is create a class that will encapsulate the functionality of the form. We can include ActiveModel::Model (Rails 4) in our class to behave like active record model, which will let us use active record validation helpers and form helpers for our form.

Let's see with an example. We have a user registration controller where we register a new user.

# app/controllers/registration_controller.rb

class RegistrationsController < ApplicationController
  respond_to :html

  def new
    @user = User.new
  end

  def create
    @user = User.new(user_params)
    if @user.save
      redirect_to @user
    else
      render :new
    end
  end

  private

  def user_params
    params.require(:user).permit(:email, :password, :name)
  end
end

And the registration form #

# app/views/registration/new.html.erb

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

  <%= f.label :email, 'Email' %>:
  <%= f.text_field :email %>

  <%= f.label :password, 'Password' %>:
  <%= f.password_field :password %>

  <%= f.label :name, 'Name' %>:
  <%= f.text_field :name %>

  <%= f.submit %>
<% end %>

Now we transform the above code to form object pattern by creating a simple class that encapsulate the logic to register user. I will put my forms files inside app/forms/ :
# app/forms/registration.rb

class Registration
  include ActiveModel::Model

  attr_reader(:email, :password, :name)

  def initialize(options={})
    @name = options[:name]
    @email = options[:email]
    @password = options[:password]
  end

  validates :email, presence: true
  validates :name, presence: true
  validates :password, presence: true

  def save
    if valid?
      create_user
    end
  end

  def persisted?
    false
  end

  private

  def create_user
    User.create({email: email, password: password, name: name})
  end
end

Change registration controller:

# app/controllers/registration_controller.rb

class RegistrationsController < ApplicationController
  respond_to :html

  def new
    @registration = Registration.new
  end

  def create
    @registration = Registration.new(registration_params)
    if @user = @registration.save
      redirect_to @user
    else
      render :new
    end
  end

  private

  def registration_params
    params.require(:registration).permit(:email, :password, :name)
  end
end

And change our registration form #

# app/views/registration/new.html.erb

<%= form_for @registration do |f| %>

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

  <%= f.label :email, 'Email' %>:
  <%= f.text_field :email %>

  <%= f.label :password, 'Password' %>:
  <%= f.password_field :password %>

  <%= f.label :first_name, 'First Name' %>:
  <%= f.text_field :first_name %>

  <%= f.submit %>
<% end %>

We can also include Virtus in our form object class. It helps to add object attributes and data type. It saves a lot of coding that may require in the form objects class. There is no need to define initialize function in our class with Virtus.

Example:

class RegistrationForm
  include ActiveModel::Model
  include Virtus.model

  attribute :name, String
  attribute :email, String
  attribute :password, String

  validates :email, presence: true
  validates :name, presence: true
  validates :password, presence: true

  def save
    if valid?
      create_user
    end
  end

  def persisted?
    false
  end

  private

  def create_user
    User.create({email: email, password: password, name: name})
  end

end

Note:

In Rails 4
  include ActiveModel::Model

In Rails 3
  extend ActiveModel::Naming
  include ActiveModel::Conversion
  include ActiveModel::Validations