comparison_validator.rb

class ComparisonValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    return if options[:allow_blank] && value.blank?
    return if options[:allow_nil] && value.nil?

    unless comparison_attribute = options[:attribute]
      raise ArgumentError, "missing attribute option"
    end

    unless operator = options[:operator]
      raise ArgumentError, "missing operator option"
    end

    comparison_value = record.public_send(comparison_attribute)

    unless value && comparison_value && value.public_send(operator, comparison_value)
      record.errors.add attribute, "must be #{operator} to #{comparison_attribute}"
    end
  end
end