ActiveRecord计数器缓存更新

我有问题 counter_cache 。假设我有三个型号。用户和文章有许多ArticleUp投票。用户可以为文章创建ArticleUpvote。

使用者

# models/user.rb
class User < ActiveRecord::Base
  has_many :upvotes, class_name: 'ArticleUpvote'

  def upvote(article)
    article.upvotes.create(user: self)
  end
end

文章

# models/article.rb
class Article < ActiveRecord::Base
  has_many :upvotes, class_name: 'ArticleUpvote'
end

条款更新投票

# models/article_upvote.rb
class ArticleUpvote < ActiveRecord::Base
  include ArticleVote

  belongs_to :article, dependent: :destroy, counter_cache: :upvotes_count
end

# models/concerns/article_vote
module ArticleVote
  extend ActiveSupport::Concern

  included do
    belongs_to :user

    validates :user, presence: true
  end
end 

未通过测试

context 'voting' do
  let(:user) { FactoryGirl.create(:user) }
  let(:article) { FactoryGirl.create(:article) }

  context '#upvote' do
    it 'adds upvote to article' do
      user.upvote(article)
      expect(article.upvotes.size).to eq 1
    end
  end
end

错误

1) User voting #upvote adds upvote to article
 Failure/Error: expect(article.upvotes.size).to eq 1

   expected: 1
        got: 0

   (compared using ==)

通过测试

只是将我的测试体更改为:

user.upvote(article)
article.upvotes.size # Added this line compared to failing version
expect(article.upvotes.size).to eq 1

或者这样做:

expect{ user.upvote(article) }.to change{ article.upvotes.size }.by 1

让测试通过。为什么会这样?