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 %s is not allowed"
end

Configuration options:

• in - An enumerable object of items that the value shouldn’t be part of
• message - Specifies a customer error message (default is: "is reserved")
• allow_nil - If set to true, skips this validation if the attribute is null (default is: false)
• allow_blank - If set to true, skips this validation if the attribute is blank (default is: false)
• 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 780
def validates_exclusion_of(*attr_names)
  configuration = { :message => ActiveRecord::Errors.default_error_messages[:exclusion], :on => :save }
  configuration.update(attr_names.extract_options!)

  enum = configuration[:in] || configuration[:within]

  raise(ArgumentError, "An object with the method include? is required must be supplied as the :in option of the configuration hash") unless enum.respond_to?("include?")

  validates_each(attr_names, configuration) do |record, attr_name, value|
    record.errors.add(attr_name, configuration[:message] % value) if enum.include?(value)
  end
end