class NeuroticException < Exception; end

module NeuroticBuilder

  @@_required_on_create = []

  def build(*args)
    instance = self.send(:new, *args)
    yield(instance) if block_given?

    @@_required_on_create.each do |attribute|
      if instance.respond_to? attribute and blank? instance.send(attribute)
        raise NeuroticException, "'#{attribute}' is blank but is required"
      end
    end

    instance
  end

private
  def blank?(attr_value)
    attr_value.respond_to?(:empty?) ? attr_value.empty? : !self
  end

  def required_on_creation(*attributes)
    @@_required_on_create = attributes
  end

  def extended(base)
    base.include(Buildable::ClassMethods)
  end
end


class Person
  extend NeuroticBuilder

  attr_accessor :first_name, :last_name, :age

  # See it! You can declare which fields are
  # required to build (instanciate) this classe.
  #
  # If any of attributes aren't filled an
  # Exception is raised.
  #
  # See the tests below for better understanding.
  required_on_creation :first_name, :age

  def initialize(*first_name)
    @first_name = first_name
  end
end


require 'test/unit'

class TestNeuroticBuilder < Test::Unit::TestCase

  def test_should_raise_a_exception_because_first_name_isnt_filled_in_creation_of_the_object

    assert_raise NeuroticException do
      person = Person.build do |p|
        p.last_name = "Senna"
        p.age       = 33
      end
    end
  end

  def test_should_not_raise_exception_because_all_of_required_attributes_are_filled
    person = nil

    assert_nothing_raised do
      person = Person.build do |p|
        p.first_name = "Ayrton"
        p.age        = 33
      end
    end

    assert_not_nil person
  end

  def test_should_not_raise_exception_because_all_of_required_attributes_are_filled_again

    assert_nothing_raised do
      person = Person.build "Ayrton" do |p| # Note
        p.last_name = "Senna"
        p.age       = 33
      end
    end
  end

end