Custom validation methods in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom validation method in Rails?

A custom validation method is a user-defined method inside a Rails model that checks if an object meets specific rules beyond built-in validations. It helps ensure data is correct before saving.

beginner

How do you tell Rails to use a custom validation method?

You use the validate :method_name syntax inside the model. Rails then calls that method during validation.

beginner

What should a custom validation method do if the data is invalid?

It should add an error message to the model's errors collection using errors.add(:attribute, "message"). This prevents saving and shows the problem.

intermediate

Why use custom validation methods instead of built-in validators?

Custom validations let you check complex or unique rules that built-in validators can't handle, like comparing two fields or checking external conditions.

beginner

Example: How to write a custom validation method that ensures a user's age is at least 18?

Inside the User model, define a method like:

def adult_age
  errors.add(:age, "must be at least 18") if age && age < 18
end
validate :adult_age

Which keyword tells Rails to run a custom validation method?

A validates_with

B validates

C validate

D validate_with

Use validate :method_name to run a custom validation method.

Where do you add error messages inside a custom validation method?

A add_error(:attribute, "message")

B errors.add(:attribute, "message")

C error.add(:attribute, "message")

D add.errors(:attribute, "message")

The correct syntax is errors.add(:attribute, "message").

What happens if a custom validation adds an error to the model?

A The model deletes itself

B The model saves anyway

C The error is ignored

D The model will not save

Adding errors prevents the model from saving until fixed.

Which of these is a reason to use a custom validation?

A To check a rule built-in validators can't handle

B To speed up saving records

C To automatically fix errors

D To skip validations

Custom validations handle special rules not covered by built-in ones.

How do you call a custom validation method named check_name?

A validate :check_name

B validates :check_name

C validate_with :check_name

D validates_with :check_name

Use validate :check_name to run the method during validation.

Explain how to create and use a custom validation method in a Rails model.

Think about where to put the method and how Rails knows to run it.

Describe why and when you would choose a custom validation method over built-in validators.

Consider what built-in validators can't do.

Practice

1. What is the main purpose of a custom validation method in a Rails model?

easy

A. To add your own rules for checking data before saving

B. To automatically generate database tables

C. To create user interface elements

D. To speed up database queries

Solution

1. Step 1: Understand validation role in Rails

   Validations check data correctness before saving to the database.

2. Step 2: Identify custom validation purpose

   Custom validations let you write your own rules beyond built-in checks.

3. Final Answer:

   To add your own rules for checking data before saving -> Option A

4. Quick Check:

   Custom validation = custom data rules [OK]

Hint: Custom validations add your own data rules [OK]

Common Mistakes:

• Confusing validations with database setup
• Thinking validations create UI elements
• Assuming validations improve query speed

2. Which is the correct way to add a custom validation method named check_age in a Rails model?

easy

A. validates :check_age

B. validate :check_age

C. validate_check_age

D. validates_with :check_age

Solution

1. Step 1: Recall syntax for custom validations

   Use validate :method_name to call a custom method.

2. Step 2: Match method name with syntax

   For method check_age, syntax is validate :check_age.

3. Final Answer:

   validate :check_age -> Option B

4. Quick Check:

   Custom validation syntax = validate :method_name [OK]

Hint: Use 'validate :method_name' for custom validations [OK]

Common Mistakes:

• Using 'validates' instead of 'validate' for custom methods
• Adding underscores in method call
• Using 'validates_with' incorrectly

3. Given this model code:

class User < ApplicationRecord
  validate :check_name

  def check_name
    errors.add(:name, "can't be blank") if name.blank?
  end
end

What happens if you try to save a User with name = ''?

medium

A. The save fails with an error on the name attribute

B. The save succeeds without errors

C. A syntax error occurs

D. The save fails but no error message is added

Solution

1. Step 1: Understand the custom validation logic

   The method adds an error if name.blank? is true (empty string).

2. Step 2: Effect on saving a User with empty name

   Since name is blank, error is added and save fails.

3. Final Answer:

   The save fails with an error on the name attribute -> Option A

4. Quick Check:

   Blank name triggers error = save fails [OK]

Hint: Blank fields trigger errors in custom validations [OK]

Common Mistakes:

• Assuming save succeeds despite errors
• Confusing blank? with nil check
• Expecting syntax errors from valid code

4. Identify the error in this custom validation method:

class Product < ApplicationRecord
  validate :check_price

  def check_price
    errors.add(:price, "must be positive") if price <= 0
  end
end

medium

A. The method name should start with 'validate_'

B. The validation should use 'validates' instead of 'validate'

C. No error; the code is correct

D. The comparison operator should be '==' instead of '<='

Solution

1. Step 1: Check method naming and usage

   Custom validation methods do not need 'validate_' prefix; 'validate :method' is correct.

2. Step 2: Verify validation logic and syntax

   Using 'validate' with method and condition 'price <= 0' is valid Ruby and Rails syntax.

3. Final Answer:

   No error; the code is correct -> Option C

4. Quick Check:

   Custom validation syntax and logic are correct [OK]

Hint: Custom validation methods do not need special prefixes [OK]

Common Mistakes:

• Adding 'validate_' prefix to method name
• Using 'validates' for custom methods
• Changing correct comparison operators

5. You want to create a custom validation that ensures a User's username is at least 5 characters and contains no spaces. Which code correctly implements this?

hard

A. validate :username_rules

def username_rules
errors.add(:username, "invalid") if username.length <= 5 && username.include?(" ")
end

B. validates :username, length: { minimum: 5 }, format: { without: /\s/ }

C. validate :username_rules

def username_rules
errors.add(:username, "invalid") unless username.length >= 5 && !username.include:(" ")
end

D. validate :username_rules

def username_rules
if username.length < 5 || username.include?(" ")
errors.add(:username, "must be 5+ chars and no spaces")
end
end

Solution

1. Step 1: Understand the validation requirements

   Username must be at least 5 characters and contain no spaces.

2. Step 2: Analyze each option's logic

   validate :username_rules
   
   def username_rules
     if username.length < 5 || username.include?(" ")
       errors.add(:username, "must be 5+ chars and no spaces")
     end
   end

   uses OR logic to add error if username is shorter than 5 or contains spaces, which matches requirements.

3. Step 3: Check other options

   validates :username, length: { minimum: 5 }, format: { without: /\s/ }

   uses built-in validations, not custom method.

   validate :username_rules
   
   def username_rules
     errors.add(:username, "invalid") unless username.length >= 5 && !username.include:(" ")
   end

   has syntax error with colon in method call.

   validate :username_rules
   
   def username_rules
     errors.add(:username, "invalid") if username.length <= 5 && username.include?(" ")
   end

   uses wrong comparison (<= 5 excludes 5).

4. Final Answer:

   validate :username_rules def username_rules if username.length < 5 || username.include?(" ") errors.add(:username, "must be 5+ chars and no spaces") end end -> Option D

5. Quick Check:

   Custom validation logic matches requirements [OK]

Hint: Check logic carefully: use OR for combined invalid conditions [OK]

Common Mistakes:

• Using AND instead of OR for combined conditions
• Confusing built-in validations with custom methods
• Wrong comparison operators causing off-by-one errors