Conditional validations in Ruby on Rails - Mini Project: Build & Apply

Conditional Validations in Rails Model

📖 Scenario: You are building a simple Rails application to manage user profiles. Some users are students, and some are teachers. You want to make sure that if a user is a student, they must provide their school_name. If the user is a teacher, they must provide their subject. This helps keep your data clean and relevant.

🎯 Goal: Create a Rails model User with conditional validations that require school_name only if the user is a student, and require subject only if the user is a teacher.

📋 What You'll Learn

Create a User model with attributes role, school_name, and subject.

Add a validation that requires school_name only when role is "student".

Add a validation that requires subject only when role is "teacher".

Use Rails built-in validation methods with conditional options.

💡 Why This Matters

🌍 Real World

Conditional validations are common in real apps where certain fields are required only in specific situations, like user roles or product types.

💼 Career

Understanding conditional validations helps you write flexible and robust Rails models, a key skill for backend web development jobs.

1

Create the User model with attributes

Create a Rails model class called User with attributes role, school_name, and subject defined as attr_accessors.

Ruby on Rails

class User
  # Define attributes role, school_name, subject
  # Your code here
end

Hint

Use attr_accessor to create getter and setter methods for the attributes.

2

Add role constants for clarity

Inside the User class, add two constants: STUDENT_ROLE set to "student" and TEACHER_ROLE set to "teacher".

Ruby on Rails

class User
  attr_accessor :role, :school_name, :subject

  # Define STUDENT_ROLE and TEACHER_ROLE constants
  # Your code here
end

Hint

Constants are written in uppercase letters and help avoid magic strings.

3

Add conditional validations for school_name and subject

Add validations inside the User class: validate presence of school_name only if role equals STUDENT_ROLE, and validate presence of subject only if role equals TEACHER_ROLE. Use the if: option with lambda functions.

Ruby on Rails

class User
  attr_accessor :role, :school_name, :subject

  STUDENT_ROLE = "student"
  TEACHER_ROLE = "teacher"

  # Add conditional validations for school_name and subject
  # Your code here
end

Hint

Use validates :attribute, presence: true, if: -> { condition } to add conditional validations.

4

Add initialize method to set attributes

Add an initialize method to the User class that accepts a hash with keys :role, :school_name, and :subject and sets the corresponding instance variables.

Ruby on Rails

class User
  attr_accessor :role, :school_name, :subject

  STUDENT_ROLE = "student"
  TEACHER_ROLE = "teacher"

  validates :school_name, presence: true, if: -> { role == STUDENT_ROLE }
  validates :subject, presence: true, if: -> { role == TEACHER_ROLE }

  # Add initialize method to set attributes from a hash
  # Your code here
end

Hint

The initialize method sets instance variables from the given hash keys.

Practice

1. What does a conditional validation in Rails do?

easy

A. Runs validation only when a specific condition is true

B. Runs validation on every save regardless of conditions

C. Disables all validations temporarily

D. Automatically fixes invalid data

Solution

1. Step 1: Understand the purpose of conditional validations

   Conditional validations allow validations to run only if certain conditions are met, making them flexible.

2. Step 2: Compare options with this purpose

   Only Runs validation only when a specific condition is true describes running validation based on a condition, matching the concept.

3. Final Answer:

   Runs validation only when a specific condition is true -> Option A

4. Quick Check:

   Conditional validation = run if condition true [OK]

Hint: Conditional validations run only when conditions are true [OK]

Common Mistakes:

• Thinking validations always run on every save
• Confusing conditional validations with disabling validations
• Believing validations fix data automatically

2. Which of the following is the correct syntax to add a conditional validation using if with a method named active??

easy

A. validates :name, presence: true if :active?

B. validates :name, presence: true, if: :active?

C. validates :name, presence: true, if active?

D. validates :name, presence: true, if => :active?

Solution

1. Step 1: Recall correct syntax for conditional validations

   Rails uses a hash key if: followed by a symbol or lambda for conditions.

2. Step 2: Match options with correct syntax

   validates :name, presence: true, if: :active? correctly uses if: :active?. Others have syntax errors or missing commas.

3. Final Answer:

   validates :name, presence: true, if: :active? -> Option B

4. Quick Check:

   Use if: :method_name for conditional validation [OK]

Hint: Use if: :method_name with a comma for conditional validations [OK]

Common Mistakes:

• Omitting comma before if
• Using if without colon or symbol
• Using wrong hash rocket syntax

3. Given this model code:

class User < ApplicationRecord
  validates :email, presence: true, unless: :guest?
  def guest?
    role == 'guest'
  end
end

What happens when you save a User with role = 'guest' and email = nil?

medium

A. Validation fails because role is not checked

B. Validation fails because email is missing

C. Raises an error due to missing email

D. Validation passes because guest users skip email validation

Solution

1. Step 1: Understand the conditional validation with unless

   The validation runs unless the guest? method returns true. If user is guest, validation is skipped.

2. Step 2: Apply to given user data

   User has role 'guest', so guest? returns true, skipping email presence validation. So saving passes despite missing email.

3. Final Answer:

   Validation passes because guest users skip email validation -> Option D

4. Quick Check:

   unless: :guest? skips validation if guest? true [OK]

Hint: unless skips validation when condition method returns true [OK]

Common Mistakes:

• Assuming validation always runs regardless of unless
• Confusing unless with if
• Expecting error instead of skip

4. Identify the error in this validation code:

validates :age, numericality: true, if: user.adult?

medium

A. No error, code is correct

B. Missing comma after numericality: true

C. Using method call instead of symbol for if condition

D. numericality should be a method, not an option

Solution

1. Step 1: Check syntax for conditional validation if option

   The if: option expects a symbol or lambda, not a method call.

2. Step 2: Analyze given code

   Here, user.adult? is a method call, which is invalid syntax in validation options.

3. Final Answer:

   Using method call instead of symbol for if condition -> Option C

4. Quick Check:

   Use symbol (:method_name), not method call, for if condition [OK]

Hint: Use :method_name symbol, not method call, in if condition [OK]

Common Mistakes:

• Calling method instead of passing symbol
• Confusing method call with symbol
• Ignoring syntax errors in validation options

5. You want to validate presence of phone_number only if the user is an admin or if the account is active. Which is the correct way to write this conditional validation?

hard

A. validates :phone_number, presence: true, if: -> { admin? || active? }

B. validates :phone_number, presence: true, if: :admin? && :active?

C. validates :phone_number, presence: true, if: [:admin?, :active?]

D. validates :phone_number, presence: true, if: admin? or active?

Solution

1. Step 1: Understand combining conditions in validations

   To combine multiple conditions, use a lambda with logical operators inside the if: option.

2. Step 2: Evaluate each option's syntax

   validates :phone_number, presence: true, if: -> { admin? || active? } uses a lambda with admin? || active?, which is correct. Others misuse symbols or syntax.

3. Final Answer:

   validates :phone_number, presence: true, if: -> { admin? || active? } -> Option A

4. Quick Check:

   Use lambda with || to combine conditions in if [OK]

Hint: Use lambda with || to combine multiple conditions in if [OK]

Common Mistakes:

• Using && or || with symbols directly
• Passing array of symbols instead of lambda
• Writing invalid Ruby syntax in if option