How to Create Custom Validation in Ruby on Rails

In Ruby on Rails, create custom validations by defining a method in your model and using validate :method_name to call it. Inside the method, add errors with errors.add(:attribute, "message") when validation fails.

Syntax

To create a custom validation in Rails, define a method in your model and use validate :method_name to run it during validation. Inside the method, use errors.add(:attribute, "error message") to add validation errors.

• validate :method_name tells Rails to run your custom method when validating.
• def method_name defines the custom validation logic.
• errors.add(:attribute, "message") adds an error to the attribute if validation fails.

ruby

class User < ApplicationRecord
  validate :email_must_be_company_domain

  def email_must_be_company_domain
    unless email&.end_with?('@company.com')
      errors.add(:email, 'must be a company email')
    end
  end
end

Example

This example shows a User model with a custom validation that checks if the email ends with '@company.com'. If not, it adds an error message to the email attribute.

ruby

class User < ApplicationRecord
  validate :email_must_be_company_domain

  def email_must_be_company_domain
    unless email&.end_with?('@company.com')
      errors.add(:email, 'must be a company email')
    end
  end
end

# Usage example in Rails console:
user = User.new(email: '[e-mail]')
user.valid? # => false
user.errors.full_messages # => ["Email must be a company email"]

user2 = User.new(email: '[e-mail]')
user2.valid? # => true
user2.errors.full_messages # => []

Output

false
["Email must be a company email"]
true
[]

Common Pitfalls

Common mistakes when creating custom validations include:

• Not calling validate :method_name, so the method never runs.
• Forgetting to add errors with errors.add, which makes validation always pass.
• Using validates instead of validate for custom methods (they are different).
• Not handling nil values safely, causing errors in the validation method.

ruby

class User < ApplicationRecord
  # Wrong: using validates instead of validate
  validates :email_must_be_company_domain

  def email_must_be_company_domain
    # No errors added, so validation always passes
    email&.end_with?('@company.com')
  end
end

# Correct way:
class User < ApplicationRecord
  validate :email_must_be_company_domain

  def email_must_be_company_domain
    unless email&.end_with?('@company.com')
      errors.add(:email, 'must be a company email')
    end
  end
end

Quick Reference

Summary tips for custom validations in Rails:

• Use validate :method_name to run your custom validation method.
• Inside the method, add errors with errors.add(:attribute, "message").
• Always handle nil values safely to avoid exceptions.
• Test validations in Rails console with valid? and errors.full_messages.

Key Takeaways

Define a method in your model and use validate :method_name to create custom validations.

Add errors inside the method with errors.add(:attribute, "message") to mark validation failures.

Always handle nil values safely in your validation methods to avoid errors.

Use Rails console to test your validations with valid? and errors.full_messages.

Do not confuse validate (for methods) with validates (for built-in validations).