validation.rb

require 'hanami/validations'

class ReservationValidation
  include Hanami::Validations
  
  predicate :before? do |other, current|
    current < other
  end

  validations do
    required(:check_in).filled
    required(:check_out).filled
    
    rule valid_dates: [:check_in, :check_out] do |check_in, check_out|
      check_in.before?(check_out)
    end
  end
end

date1 = Date.new(2020,1,20)
date2 = Date.new(2010,2,1)

ReservationValidation.new(check_in: date1, check_out: date2).validate.success?
# => true

ReservationValidation.new(check_in: date2, check_out: date1).validate.success?
# => false