undefined method 'i18n_scope' raised by ActiveModel::Errors when validated model is not extended with ActiveModel::Translation. Incorrect documentation about ActiveModel::Errors class.

This is extract from ActiveModel::Errors documentation:

  # A minimal implementation could be:
  # 
  #   class Person
  #   
  #     # Required dependency for ActiveModel::Errors
  #     extend ActiveModel::Naming
  # 
  #     def initialize
  #       @errors = ActiveModel::Errors.new(self)
  #     end
  #   
  #     attr_accessor :name
  #     attr_reader   :errors
  #   
  #     def validate!
  #       errors.add(:name, "can not be nil") if name == nil
  #     end
  #   
  #     # The following methods are needed to be minimally implemented
  #
  #     def read_attribute_for_validation(attr)
  #       send(attr)
  #     end
  #   
  #     def ErrorsPerson.human_attribute_name(attr, options = {})
  #       attr
  #     end
  #   
  #     def ErrorsPerson.lookup_ancestors
  #       [self]
  #     end
  #   
  #   end
  #
  # The last three methods are required in your object for Errors to be
  # able to generate error messages correctly and also handle multiple
  # languages.  Of course, if you extend your object with ActiveModel::Translations
  # you will not need to implement the last two.  Likewise, using
  # ActiveModel::Validations will handle the validation related methods
  # for you.

Problem 1: I think it should state that Person.human_attribute_name and Perons.lookup_ancestors are necessary. They should be defined for Person class, not for ErrorsPerson class.

Problem 2: Implementing those methods on Person class is not enough. When you do so it leads to the exception:

undefined method `i18n_scope' for Person:Class

activemodel (3.0.0.beta4) lib/active_model/errors.rb:261:in `block in generate_message'
activemodel (3.0.0.beta4) lib/active_model/errors.rb:260:in `map'
activemodel (3.0.0.beta4) lib/active_model/errors.rb:260:in `generate_message'
activemodel (3.0.0.beta4) lib/active_model/errors.rb:182:in `add'

Problem 3: To walk around second problem the class should be extended with ActiveModel::Translation, not ActiveModel::Translations as the documentation states.