date_time_validator.rb

class DateTimeValidator < ActiveModel::EachValidator
  CHECKS = {
    after: :>,
    before: :<,
    after_or_equal_to: :>=,
    before_or_equal_to: :<=
  }

  def check_validity!
    keys = CHECKS.keys
    options.slice(*keys).each do |option, value|
      unless value.is_a?(Symbol) || value.is_a?(Time)
        raise ArgumentError, ":#{option} must be a Symbol or a valid Time"
      end
    end
  end

  def validate_each(record, attr_name, value)
    return if options[:allow_nil] && value.nil?
    options.slice(*CHECKS.keys).each do |option, option_value|
      case option_value
      when Symbol
        option_value = record.send(option_value)
      when Time
        option_value = option_value
      end

      unless value.send(CHECKS[option], option_value)
        record.errors[attr_name] << "must be #{option.to_s.humanize.downcase} #{option_value}"
      end
    end
  end
end