Challenge 2: A Custom Validation — Solution

class Event < ApplicationRecord
  validate :end_time_after_start_time

  private

  def end_time_after_start_time
    return if start_time.blank? || end_time.blank?

    if end_time <= start_time
      errors.add(:end_time, "must be after the start time")
    end
  end
end

=begin
Notes:
- No single built-in validator (presence, length, numericality, etc.) can
  express "one field must come after another field" -- that requires
  comparing two attributes against each other, which is exactly what a
  custom validation method is for.
- validate :end_time_after_start_time (singular "validate") registers a
  custom instance method, distinct from validates (plural), which is
  reserved for the built-in attribute validators shown in Challenge 1.
- return if start_time.blank? || end_time.blank? guards against running
  the comparison at all when either field is missing -- letting the
  ordinary presence validators (not shown here, but would typically also
  be present) handle that case separately, rather than raising a
  confusing NoMethodError trying to compare nil to nil.
- errors.add(:end_time, "...") attaches the error specifically to
  end_time, which matters later for form rendering (Chapter 8) — Rails
  can highlight exactly which field the error belongs to.
=end