为什么在我的单元测试中查找(:last)失败?

我在Rails中有一对多的关系:

class User < ActiveRecord::Base
  has_many :activities, :order => "added_at DESC"


class Activity < ActiveRecord::Base
  belongs_to :user

我有一个活动的方法:

def self.test_message(user, message)
  user.activities << Activity.create do |activity|
    activity.message = message
    activity.added_at = Time.now
  end    
end

以及以下单元测试:

require 'test_helper'

class ActivityTest < ActiveSupport::TestCase

  def test_test_message
    #From fixture
    alice = User.find_by_name("alice")
    assert_equal 0, alice.activities.count

    Activity.test_message(alice, "Hello")
    assert_equal 1, alice.activities.count

    Activity.test_message(alice, "Goodbye")
    assert_equal 2, alice.activities.count
    assert_equal "Hello", alice.activities.find(:first).message

    #The following line fails with: Goodbye expected but was Hello
    assert_equal "Goodbye", alice.activities.find(:last).message,
    acts = alice.activities
    assert_equal 2, acts.count
    assert_equal "Goodbye", acts[1].message
  end
end

在指定的线路上失败了,但我不知道为什么。

另外,使用activities.find(:last)在使用开发环境时有效,但仅在测试环境下失败。我已经删除并重建了数据库。