Does this a design gotcha? Maybe more likely is a bug.
I have read the code, and found that only :is, :maximum, :mininum will accept the :message option. Below is the related code.
442: def validates_length_of(*attrs) 443: # Merge given options with defaults. 444: options = { 445: :too_long => ActiveRecord::Errors.default_error_messages[:too_long], 446: :too_short => ActiveRecord::Errors.default_error_messages[:too_short], 447: :wrong_length => ActiveRecord::Errors.default_error_messages[:wrong_length] 448: }.merge(DEFAULT_VALIDATION_OPTIONS) 449: options.update(attrs.pop.symbolize_keys) if attrs.last.is_a?(Hash) 450: 451: # Ensure that one and only one range option is specified. 452: range_options = ALL_RANGE_OPTIONS & options.keys 453: case range_options.size 454: when 0 455: raise ArgumentError, 'Range unspecified. Specify the :within, :maximum, :minimum, or :is option.' 456: when 1 457: # Valid number of options; do nothing. 458: else 459: raise ArgumentError, 'Too many range options specified. Choose only one.' 460: end 461: 462: # Get range option and value. 463: option = range_options.first 464: option_value = options[range_options.first] 465: 466: case option 467: when :within, :in 468: raise ArgumentError, ":#{option} must be a Range" unless option_value.is_a?(Range) 469: 470: too_short = options[:too_short] % option_value.begin 471: too_long = options[:too_long] % option_value.end 472: 473: validates_each(attrs, options) do |record, attr, value> 474: if value.nil? or value.split(//).size < option_value.begin 475: record.errors.add(attr, too_short) 476: elsif value.split(//).size > option_value.end 477: record.errors.add(attr, too_long) 478: end 479: end 480: when :is, :minimum, :maximum 481: raise ArgumentError, ":#{option} must be a nonnegative Integer" unless option_value.is_a?(Integer) and option_value >= 0 482: 483: # Declare different validations per option. 484: validity_checks = { :is => "==", :minimum => ">=", :maximum => "<=" } 485: message_options = { :is => :wrong_length, :minimum => :too_short, :maximum => :too_long } 486: 487: message = (options[:message] || options[message_options[option]]) % option_value 488: 489: validates_each(attrs, options) do |record, attr, value> 490: if value.kind_of?(String) 491: record.errors.add(attr, message) unless ! value.nil? and value.split(//).size.method(validity_checks[option]) [option_value] 492: else 493: record.errors.add(attr, message) unless ! value.nil? and value.size.method(validity_checks[option]) [option_value] 494: end 495: end 496: end 497: end