如何在保存Rails之前验证对象？

Question

我在Rails中非常新，我有以下问题。我有这个类Url，它有自己的验证：

class Url
  validates :ipaddress, :format => {
    :with => Resolv::IPv4::Regex,
    :message => 'Deve ser um endereco IP valido'
  }

  validates :domain, :format => {
    :with => URI.regexp,
    :message => 'Deve ser um dominio valido'
  }
end

我有另一个Link的班级Url。验证是：

class Link
  validates :path, :format => {
    :with => /\A[\S]+\Z/,
    :message => 'Deve conter uma path valida'
  }
end

但问题是当我运行此测试时：

test 'test a link with a valid path' do
  link = Link.new()
  link.url = Url.new()
  link.path = 'this/is/my/favorite/path'
  assert_equal(true, link.valid?, 'Test a valid path')
end

测试表明一切正常，但未验证URL。我怎样才能强制Rails验证URL呢？