Conditional validations in Ruby on Rails

Introduction

Conditional validations let you check data only when certain rules apply. This helps keep your app flexible and accurate.

You want to validate a field only if another field has a specific value.

You need to check a user's input only when they choose a certain option.

You want to skip validation in some cases to allow different workflows.

You want to validate a field only if the record is new or being updated.

Syntax

Ruby on Rails

validates :field_name, presence: true, if: :condition_method

# or
validates :field_name, presence: true, unless: :condition_method

# condition_method is a method that returns true or false

The if option runs validation only when the method returns true.

The unless option runs validation only when the method returns false.

Examples

This validates email only if the user is signed in.

Ruby on Rails

validates :email, presence: true, if: :email_required?

def email_required?
  user_signed_in?
end

This skips age validation if the user is an admin.

Ruby on Rails

validates :age, numericality: { greater_than: 18 }, unless: :admin?

def admin?
  role == 'admin'
end

This uses a lambda to check if the record is new before validating password presence.

Ruby on Rails

validates :password, presence: true, if: -> { new_record? }

# Uses a lambda to validate password only when creating a new record.

Sample Program

This User model requires phone_number only if contact_method is 'phone'. The example shows validation fails when phone_number is empty but contact_method is 'phone'. It passes when contact_method is 'email' even if phone_number is empty.

Ruby on Rails

class User < ApplicationRecord
  validates :phone_number, presence: true, if: :contact_by_phone?

  def contact_by_phone?
    contact_method == 'phone'
  end
end

# Example usage:
user1 = User.new(contact_method: 'phone', phone_number: '')
user2 = User.new(contact_method: 'email', phone_number: '')

puts user1.valid? # false because phone_number is required
puts user2.valid? # true because phone_number not required

Important Notes

Use simple method names for conditions to keep code readable.

Lambdas can be used for inline conditions without defining separate methods.

Conditional validations help avoid unnecessary errors and improve user experience.

Summary

Conditional validations run only when specific conditions are met.

Use if or unless with methods or lambdas.

This makes your validations flexible and context-aware.

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