class ActiveModel::Validations::AcceptanceValidator::LazilyDefineAttributes

def included(klass)
  @lock = Mutex.new
  mod = self
  define_method(:respond_to_missing?) do |method_name, include_private = false|
    mod.define_on(klass)
    super(method_name, include_private) || mod.matches?(method_name)
  end
  define_method(:method_missing) do |method_name, *args, &block|
    mod.define_on(klass)
    if mod.matches?(method_name)
      send(method_name, *args, &block)
    else
      super(method_name, *args, &block)
    end
  end
end