class ActiveModel::Validations::ExclusionValidator

def check_validity!
  raise ArgumentError, "An object with the method include? is required must be supplied as the " <<
                       ":in option of the configuration hash" unless options[:in].respond_to?(:include?)
end

def validate_each(record, attribute, value)
  if options[:in].include?(value)
    record.errors.add(attribute, :exclusion, options.except(:in).merge!(:value => value))
  end
end