Let's say you need to add a validation for the
birthdate column so that the User's age is between 1 and 3 (must be a toddler).
In your User model:
validate :toddler_age def toddler_age birthdate_within_range = ((4.years.ago - 1.day)..1.year.ago).cover?(birthdate) errors.add(:birthdate, 'must be 1-3 years old') unless birthdate_within_range end