Dynamically extending an AR class with a validation

wow.rb

# This works like a dream!
module FooValidator
  attr_accessor :foo
  def FooValidator.extended(base)    
    base.singleton_class.validates_presence_of :foo
  end
end

u = User.first
u.extend(FooValidator)

u.valid?
u.errors.full_messages
["Foo can't be blank"]

# However, now this throws an error

User.first.valid?

# NameError: undefined local variable or method `_callback_before_12716' for #<User:0x0000000feec938>