validates_length_of

Validates that the specified attributes match the length restrictions supplied. Only one constraint option can be used at a time apart from :minimum and :maximum that can be combined together:

class Person < ActiveRecord::Base
  validates_length_of :first_name, maximum: 30
  validates_length_of :last_name, maximum: 30, message: "less than 30 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 :zip_code, minimum: 5, too_short: 'please enter at least 5 characters'
  validates_length_of :smurf_leader, is: 4, message: "papa is spelled with 4 characters... don't play me."
  validates_length_of :words_in_essay, minimum: 100, too_short: 'Your essay must be at least 100 words.'

  private
    def words_in_essay
      essay.scan(/\w+/)
    end
end

Constraint options:

• :minimum - The minimum size of the attribute.

• :maximum - The maximum size of the attribute. Allows nil by default if not used with :minimum.

• :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.

Other options:

• :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 %{count} characters)”).

• :too_short - The error message if the attribute goes under the minimum (default is: “is too short (minimum is %{count} 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 %{count} 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.

There is also a list of default options supported by every validator: :if, :unless, :on, and :strict. See ActiveModel::Validations::ClassMethods#validates for more information.