Ruby on Rails:如何在数据库中总结这些元素？

Question

我有一个video_votes表,其中所有的投票都有一个名为value的列设置为1或-1.我想总结视频投票的所有价值并显示净投票数.首先,我应该如何总结,其次,我应该将这个值存储在我的视频表中吗？如果是这样,怎么样？

Answer 1

我会从这开始,直到性能成为一个问题:

class Video < AR::Base
  has_many :video_votes

  def vote_sum
    video_votes.sum(:value)
  end
end

class VideoVote < AR::Base
  belongs_to :video
  validates_inclusion_of :value, :in => [-1,1]
end

一旦性能成为一个问题,我想缓存总和值,我可能会做这样的事情:

class Video < AR::Base
  has_many :video_votes

  # Override vote_sum attribute to get the db count if not stored in the db yet. 
  # The alternative is that you could remove this method and have the field
  # populated by a migration.
  def vote_sum
    read_attribute(:vote_sum) || video_votes.sum(:value)
  end
end

class VideoVote < AR::Base
  belongs_to :video
  validates_inclusion_of :value, :in => [-1,1]

  after_create :update_video_vote_sum

private

  def update_video_vote_sum
    video.update_attributes(:vote_sum => video.vote_sum + value)
  end
end

查看关于"覆盖默认访问者"的AR文档(向下滚动一下) http://ar.rubyonrails.org/classes/ActiveRecord/Base.html