Form object pattern in Ruby on Rails - Mini Project: Build & Apply

Form Object Pattern in Rails

📖 Scenario: You are building a Rails app where users can register with their name and email. Instead of saving directly to the User model, you want to use a form object to handle validations and data processing cleanly.

🎯 Goal: Create a form object class that collects name and email, validates presence of both, and provides a save method to create a User record if valid.

📋 What You'll Learn

Create a form object class called RegistrationForm

Add name and email attributes to the form object

Validate presence of name and email in the form object

Implement a save method that creates a User if validations pass

💡 Why This Matters

🌍 Real World

Form objects help keep Rails controllers and models clean by handling complex form logic in one place.

💼 Career

Understanding form objects is useful for Rails developers to write maintainable and testable code in real applications.

1

Create the RegistrationForm class with attributes

Create a Ruby class called RegistrationForm that has attr_accessor for name and email.

Ruby on Rails

class RegistrationForm
  # Your code here
end

Hint

Use attr_accessor to create getter and setter methods for name and email.

2

Add validations for presence of name and email

Inside the RegistrationForm class, include ActiveModel::Model and add validations to require presence of name and email.

Ruby on Rails

class RegistrationForm
  attr_accessor :name, :email
  # Your code here
end

Hint

Include ActiveModel::Model to get validation features, then use validates for presence checks.

3

Implement the save method to create a User

Add a save method to RegistrationForm that returns false if the form is invalid. If valid, it creates a new User with name and email and saves it.

Ruby on Rails

class RegistrationForm
  include ActiveModel::Model
  attr_accessor :name, :email

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

  # Your code here
end

Hint

Use valid? to check validations, then create a User with the form data.

4

Use the RegistrationForm in a controller action

In a controller action, create a new RegistrationForm instance with name and email from params, call save, and handle success or failure accordingly.

Ruby on Rails

def create
  # Your code here
end

Hint

Instantiate the form with params, call save, then redirect or render based on result.

Practice

1. What is the main purpose of using a Form Object in Rails?

easy

A. To speed up database queries

B. To replace ActiveRecord models completely

C. To separate form validation and logic from database models

D. To generate HTML forms automatically

Solution

1. Step 1: Understand the role of form objects

   Form objects are used to keep form-related logic and validations separate from the database models.

2. Step 2: Compare with other options

   They do not replace ActiveRecord models or speed up queries, nor do they generate HTML forms automatically.

3. Final Answer:

   To separate form validation and logic from database models -> Option C

4. Quick Check:

   Form object purpose = Separation of form logic [OK]

Hint: Form objects separate form logic from models [OK]

Common Mistakes:

• Thinking form objects replace database models
• Assuming form objects speed up queries
• Confusing form objects with view helpers

2. Which module is commonly included in a Rails form object to provide model-like behavior?

easy

A. ActiveModel::Model

B. ActiveRecord::Base

C. ActionController::Base

D. ActiveSupport::Concern

Solution

1. Step 1: Identify the module for model-like behavior without DB

   The ActiveModel::Model module allows plain Ruby objects to behave like ActiveRecord models, including validations.

2. Step 2: Eliminate other options

   ActiveRecord::Base is for database models, ActionController::Base is for controllers, and ActiveSupport::Concern is for mixins.

3. Final Answer:

   ActiveModel::Model -> Option A

4. Quick Check:

   Form object module = ActiveModel::Model [OK]

Hint: Form objects include ActiveModel::Model for validations [OK]

Common Mistakes:

• Using ActiveRecord::Base in form objects
• Confusing controller modules with model modules
• Forgetting to include any module

3. Given this form object code, what will user_form.valid? return if email is blank?

class UserForm
  include ActiveModel::Model
  attr_accessor :email
  validates :email, presence: true
end

user_form = UserForm.new(email: '')
user_form.valid?

medium

A. true

B. false

C. nil

D. Raises an error

Solution

1. Step 1: Check validation rules

   The form object requires email to be present (not blank).

2. Step 2: Evaluate given input

   The email is an empty string, which fails the presence validation, so valid? returns false.

3. Final Answer:

   false -> Option B

4. Quick Check:

   Blank email fails presence validation = false [OK]

Hint: Presence validation fails on blank string [OK]

Common Mistakes:

• Assuming blank string passes presence validation
• Expecting valid? to raise error on blank
• Confusing nil and empty string behavior

4. Identify the error in this form object code:

class OrderForm
  include ActiveModel::Model
  attr_accessor :quantity
  validates :quantity, numericality: { greater_than: 0 }

  def save
    if valid?
      Order.create(quantity: quantity)
    end
  end
end

medium

A. Missing return value in save method

B. Order model is not defined

C. quantity is a string, needs conversion before validation

D. No error, code is correct

Solution

1. Step 1: Verify syntax and structure

   The code properly includes ActiveModel::Model, defines attr_accessor, and sets up numericality validation. Syntax is valid.

2. Step 2: Confirm behavior

   Numericality validation internally handles string-to-number conversion (e.g., '5' validates as > 0). The save method returns the Order instance (truthy) on success or nil/falsey otherwise, which works in conditionals.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   Numericality handles strings; save pattern acceptable [OK]

Hint: Rails numericality validator handles numeric strings automatically [OK]

Common Mistakes:

• Thinking strings need manual conversion for numericality
• Overthinking save method's return value
• Assuming undefined models cause errors in examples

5. You want to create a form object that handles user registration with email, password, and password_confirmation. Which approach correctly ensures password confirmation validation and saves the user if valid?

class RegistrationForm
  include ActiveModel::Model
  attr_accessor :email, :password, :password_confirmation

  validates :email, presence: true
  validates :password, presence: true, confirmation: true

  def save
    if valid?
      User.create(email: email, password: password)
    end
  end
end

hard

A. This code correctly validates and saves the user with password confirmation

B. Missing manual check for password_confirmation equality

C. Should use validates_confirmation_of :password_confirmation instead

D. Must call super in save method to save user

Solution

1. Step 1: Check validations for password confirmation

   The confirmation: true option on password automatically validates that password_confirmation matches.

2. Step 2: Verify save method logic

   The save method calls valid? and creates the user if valid, which is the correct pattern.

3. Final Answer:

   This code correctly validates and saves the user with password confirmation -> Option A

4. Quick Check:

   Password confirmation validation works with confirmation: true [OK]

Hint: Use confirmation: true for password confirmation validation [OK]

Common Mistakes:

• Manually checking password_confirmation equality
• Using deprecated validates_confirmation_of syntax
• Forgetting to call valid? before save