Conditional validations in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when conditional validation is triggered?

Consider this Rails model snippet:

class User < ApplicationRecord
  validates :email, presence: true, if: :email_required?

  def email_required?
    age >= 18
  end
end

What is the validation behavior when a User is 17 years old and email is blank?

A Validation passes because email presence is only checked if age is 18 or more.

B Validation passes only if email is blank and age is exactly 17.

C Validation fails because age is less than 18 but email is blank.

D Validation fails because email must always be present.

💡 Hint

Look at the condition method email_required? and when it returns true.

The validation for email presence runs only if email_required? returns true. Since age is 17, email_required? returns false, so email presence is not validated.

📝 Syntax

intermediate

Identify the correct syntax for conditional validation using :unless

Which option correctly uses :unless to skip validation when the user is an admin?

class User < ApplicationRecord
  # Validate presence of password unless user is admin
end

A validates :password, presence: true, unless: admin?

B validates :password, presence: true, unless: :admin?

C validates :password, presence: true, unless: admin

D validates :password, presence: true, unless: -> { admin? }

💡 Hint

The :unless option expects a symbol referencing a method name.

The :unless option takes a symbol for a method name that returns true or false. Option B correctly uses :admin? as a symbol.

🔧 Debug

advanced

Why does this conditional validation never run?

Given this model:

class Product < ApplicationRecord
  validates :price, numericality: true, if: :on_sale

  def on_sale
    sale == true
  end
end

Why might the price validation never trigger even when sale is true?

A The method on_sale should be named on_sale? with a question mark.

B The if option expects a symbol, but :on_sale is a method returning a boolean, so it works fine.

C The validation needs to use unless instead of if to work correctly.

D The sale attribute might be a string 'true' instead of boolean true, causing on_sale to return false.

💡 Hint

Check the data type of the sale attribute and how Ruby compares true with 'true'.

If sale is stored as a string 'true', the comparison sale == true returns false, so on_sale returns false and validation does not run.

❓ state_output

advanced

What is the validation error output for this conditional validation?

Consider this model:

class Order < ApplicationRecord
  validates :shipping_address, presence: true, if: -> { delivery_required }

  def delivery_required
    delivery == 'yes'
  end
end

order = Order.new(delivery: 'yes', shipping_address: '')
order.valid?
order.errors.full_messages

What is the output of order.errors.full_messages?

A [] (empty array)

B ["Delivery must be yes"]

C ["Shipping address can't be blank"]

D ["Shipping address is invalid"]

💡 Hint

The validation runs only if delivery_required returns true.

Since delivery is 'yes', delivery_required returns true, so shipping_address presence is validated. It is blank, so error is added.

🧠 Conceptual

expert

Which conditional validation approach is best for complex logic?

You want to validate an attribute only if multiple conditions are met, including checking user role, time of day, and a feature flag. Which approach is best in Rails?

A Use a single method that returns true or false combining all conditions, then use if: :method_name.

B Chain multiple validations each with simple if conditions for each check separately.

C Use a lambda with all conditions inside for the if option.

D Use validates with unless option negating all conditions.

💡 Hint

Think about maintainability and clarity for complex conditions.

A single method encapsulating all logic keeps validation clean and easy to maintain. Lambdas can be used but may become hard to read if complex.

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