Validates that the specified attribute matches the length restrictions supplied. Only one option can be used at a time:

class Person < ActiveRecord::Base
  validates_length_of :first_name, :maximum=>30
  validates_length_of :last_name, :maximum=>30, :message=>"less than %d if you don't mind"
  validates_length_of :fax, :in => 7..32, :allow_nil => true
  validates_length_of :phone, :in => 7..32, :allow_blank => true
  validates_length_of :user_name, :within => 6..20, :too_long => "pick a shorter name", :too_short => "pick a longer name"
  validates_length_of :fav_bra_size, :minimum=>1, :too_short=>"please enter at least %d character"
  validates_length_of :smurf_leader, :is=>4, :message=>"papa is spelled with %d characters... don't play me."
end

Configuration options:

• minimum - The minimum size of the attribute
• maximum - The maximum size of the attribute
• is - The exact size of the attribute
• within - A range specifying the minimum and maximum size of the attribute
• in - A synonym(or alias) for :within
• allow_nil - Attribute may be nil; skip validation.
• allow_blank - Attribute may be blank; skip validation.
• too_long - The error message if the attribute goes over the maximum (default is: "is too long (maximum is %d characters)")
• too_short - The error message if the attribute goes under the minimum (default is: "is too short (min is %d characters)")
• wrong_length - The error message if using the :is method and the attribute is the wrong size (default is: "is the wrong length (should be %d characters)")
• message - The error message to use for a :minimum, :maximum, or :is violation. An alias of the appropriate too_long/too_short/wrong_length message
• on - Specifies when this validation is active (default is :save, other options :create, :update)
• if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
• unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.

Source Code

# File active_record/validations.rb, line 557
def validates_length_of(*attrs)
  # Merge given options with defaults.
  options = {
    :too_long     => ActiveRecord::Errors.default_error_messages[:too_long],
    :too_short    => ActiveRecord::Errors.default_error_messages[:too_short],
    :wrong_length => ActiveRecord::Errors.default_error_messages[:wrong_length]
  }.merge(DEFAULT_VALIDATION_OPTIONS)
  options.update(attrs.extract_options!.symbolize_keys)

  # Ensure that one and only one range option is specified.
  range_options = ALL_RANGE_OPTIONS & options.keys
  case range_options.size
    when 0
      raise ArgumentError, 'Range unspecified.  Specify the :within, :maximum, :minimum, or :is option.'
    when 1
      # Valid number of options; do nothing.
    else
      raise ArgumentError, 'Too many range options specified.  Choose only one.'
  end

  # Get range option and value.
  option = range_options.first
  option_value = options[range_options.first]

  case option
    when :within, :in
      raise ArgumentError, ":#{option} must be a Range" unless option_value.is_a?(Range)

      too_short = options[:too_short] % option_value.begin
      too_long  = options[:too_long]  % option_value.end

      validates_each(attrs, options) do |record, attr, value|
        if value.nil? or value.split(//).size < option_value.begin
          record.errors.add(attr, too_short)
        elsif value.split(//).size > option_value.end
          record.errors.add(attr, too_long)
        end
      end
    when :is, :minimum, :maximum
      raise ArgumentError, ":#{option} must be a nonnegative Integer" unless option_value.is_a?(Integer) and option_value >= 0

      # Declare different validations per option.
      validity_checks = { :is => "==", :minimum => ">=", :maximum => "<=" }
      message_options = { :is => :wrong_length, :minimum => :too_short, :maximum => :too_long }

      message = (options[:message] || options[message_options[option]]) % option_value

      validates_each(attrs, options) do |record, attr, value|
        if value.kind_of?(String)
          record.errors.add(attr, message) unless !value.nil? and value.split(//).size.method(validity_checks[option])[option_value]
        else
          record.errors.add(attr, message) unless !value.nil? and value.size.method(validity_checks[option])[option_value]
        end
      end
  end
end