Custom validation methods in Ruby on Rails

Introduction

Custom validation methods let you check if your data is correct in ways built-in checks can't. They help keep your app's information clean and reliable.

When you need to check a rule that Rails does not provide by default.

When validating a combination of fields together.

When you want to add a special error message for a unique condition.

When you want to reuse a validation logic inside your model.

When you want to keep your validation code organized and clear.

Syntax

Ruby on Rails

class ModelName < ApplicationRecord
  validate :custom_validation_method

  def custom_validation_method
    if some_condition_is_not_met
      errors.add(:attribute_name, "custom error message")
    end
  end
end

The method name after validate is your custom check.

Use errors.add to add an error message to a specific attribute.

Examples

This checks if the user's age is less than 18 and adds an error if so.

Ruby on Rails

class User < ApplicationRecord
  validate :must_be_adult

  def must_be_adult
    if age.present? && age.to_i < 18
      errors.add(:age, "must be at least 18 years old")
    end
  end
end

This ensures the product price is a positive number.

Ruby on Rails

class Product < ApplicationRecord
  validate :price_must_be_positive

  def price_must_be_positive
    errors.add(:price, "must be greater than zero") if price.to_f <= 0
  end
end

This validates that the start date is earlier than the end date.

Ruby on Rails

class Booking < ApplicationRecord
  validate :start_date_before_end_date

  def start_date_before_end_date
    if start_date.present? && end_date.present? && start_date > end_date
      errors.add(:start_date, "must be before end date")
    end
  end
end

Sample Program

This Event model checks that the name is not empty and starts with a capital letter. The example creates an event with a lowercase name and shows the validation errors.

Ruby on Rails

class Event < ApplicationRecord
  validate :name_cannot_be_blank_and_must_start_with_capital

  def name_cannot_be_blank_and_must_start_with_capital
    if name.blank?
      errors.add(:name, "can't be blank")
    elsif name[0] != name[0].upcase
      errors.add(:name, "must start with a capital letter")
    end
  end
end

# Example usage:
event = Event.new(name: "party")
valid = event.valid?
errors = event.errors.full_messages

puts "Is event valid? #{valid}"
puts "Errors: #{errors.join(', ')}"

Important Notes

Custom validations run when you call valid? or try to save the record.

Always add errors to the errors object to stop saving invalid data.

You can add errors to specific attributes or to the base object for general errors.

Summary

Custom validation methods let you write your own rules for data checks.

Use validate :method_name and define the method to add errors.

This keeps your app's data clean and your validation logic clear.

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