Conditional validations in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a conditional validation in Rails?

A conditional validation runs only when a specific condition is true. It helps check data only in certain cases, like validating a field only if another field has a certain value.

beginner

How do you use :if option in Rails validations?

You add :if with a method name, proc, or lambda that returns true or false. The validation runs only if it returns true.

beginner

Example: Validate presence of nickname only if user_type is 'guest'. How to write this?

Use validates :nickname, presence: true, if: -> { user_type == 'guest' }. This means nickname must be present only when user_type equals 'guest'.

intermediate

What is the difference between :if and :unless in Rails validations?

:if runs validation when condition is true. :unless runs validation when condition is false. They are opposites.

intermediate

Can you use multiple conditions in Rails validations? How?

Yes, you can combine :if and :unless or use methods that check multiple conditions. The validation runs only if all conditions are met.

Which option runs a validation only when a condition is true?

A :if

B :unless

C :only_if

D :when

The :if option runs the validation only if the given condition returns true.

What does this validation do? validates :email, presence: true, unless: :admin?

A Validates email presence only if user is not admin

B Validates email presence only if user is admin

C Always validates email presence

D Never validates email presence

The :unless option skips validation if the method admin? returns true, so it validates only if user is not admin.

How can you write a conditional validation using a method?

A validates :name, presence: true, if: method_name

B validates :name, presence: true, if: method_name()

C validates :name, presence: true, if: 'method_name'

D validates :name, presence: true, if: :method_name

You pass the method name as a symbol to :if, like if: :method_name, so Rails calls it to check the condition.

What type of object can you use with :if for conditional validation?

A Only symbols

B Symbols, procs, or lambdas

C Only strings

D Only booleans

:if accepts symbols (method names), procs, or lambdas that return true or false.

If you want a validation to run only when two conditions are true, what should you do?

A Use :if and :unless together

B Use two separate validations

C Combine conditions in one method or lambda for :if

D Use :only_if option

Combine the conditions inside one method or lambda and pass it to :if so validation runs only if all conditions are true.

Explain how to use conditional validations in Rails with examples.

Think about when you want validations to run only sometimes.

Describe the difference between :if and :unless in Rails validations and when to use each.

They are opposites controlling when validations run.

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