class SlugValidator < ActiveModel::Validator
SLUG_REGEX = /\A[a-z0-9]+(?:-[a-z0-9]+)*\z/
def validate(record)
slug = record.send(options[:attribute] || :slug)
if slug.blank?
record.errors.add(:slug, 'cannot be blank')
return
end
unless slug.match?(SLUG_REGEX)
record.errors.add(:slug, 'must contain only lowercase letters, numbers, and hyphens')
end
if slug.length < options[:minimum] || slug.length > options[:maximum]
record.errors.add(:slug, "must be between #{options[:minimum]} and #{options[:maximum]} characters")
end
# Check for reserved words
reserved_slugs = %w[admin api new edit delete]
if reserved_slugs.include?(slug)
record.errors.add(:slug, 'is reserved and cannot be used')
end
end
end
class Post < ApplicationRecord
validates :title, presence: true, length: { minimum: 5, maximum: 100 }
validates :slug, uniqueness: { scope: :author_id }
validates_with SlugValidator, attribute: :slug, minimum: 3, maximum: 60
# Custom validation method
validate :published_at_in_past, if: :published?
before_validation :generate_slug, if: :title_changed?
private
def published_at_in_past
if published_at.present? && published_at > Time.current
errors.add(:published_at, 'cannot be in the future')
end
end
def generate_slug
self.slug = title.parameterize
end
end
Custom validators encapsulate complex validation rules that go beyond built-in validators. I create validator classes for business logic like email format verification, slug uniqueness, or credit card validation. Custom validators inherit from ActiveModel::Validator and implement a validate method that adds errors to the record. They're reusable across models and testable in isolation. For simple one-off validations, I use validate with a method name. Complex conditional validations use if/unless options. I keep validators focused—each validates one concern. Error messages use I18n for internationalization. This pattern keeps models clean while enforcing business rules consistently.
