validates_exclusion_of

Validates that the value of the specified attribute is not in a particular enumerable object.

class Person < ActiveRecord::Base
  validates_exclusion_of :username, in: %w( admin superuser ), message: "You don't belong here"
  validates_exclusion_of :age, in: 30..60, message: 'This site is only for under 30 and over 60'
  validates_exclusion_of :format, in: %w( mov avi ), message: "extension %{value} is not allowed"
  validates_exclusion_of :password, in: ->(person) { [person.username, person.first_name] },
                         message: 'should not be the same as your username or first name'
  validates_exclusion_of :karma, in: :reserved_karmas
end

Configuration options:

• :in - An enumerable object of items that the value shouldn’t be part of. This can be supplied as a proc, lambda, or symbol which returns an enumerable. If the enumerable is a numerical, time, or datetime range the test is performed with Range#cover?, otherwise with include?. When using a proc or lambda the instance under validation is passed as an argument.

• :within - A synonym(or alias) for :in Range#cover?, otherwise with include?.

• :message - Specifies a custom error message (default is: “is reserved”).

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