Ruby on Rails：如何在数据库中总结这些元素？

Question

I have a video_votes table with all the votes with a column called value set to 1 or -1. I want to sum up all the values of the video's votes and display that net vote count. First, how should I sum this up, and second, should I store this value in my video table? If so, how?

Answer 1

I would start with this until performance became an issue:

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

Once performance became an issue and I wanted to cache the summed value I might do something like this:

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

Check out the AR documentation on "Overwriting default accessors" (scroll down a bit) http://ar.rubyonrails.org/classes/ActiveRecord/Base.html

Answer 2

In your Video model:

def total_votes
  self.votes.sum(:value)
end

So an example might be:

@video.total_votes

Answer 3

I'm to sum 2 fields on the level a model:

  def update_sum_times
    update_attribute(:sum_of_fields, calc_two_dates)
  end

  def calc_two_dates
    date_one + date_two
  end

Answer 4

Use ActiveRecord's sum method.

VideoVote.sum('value')

You shouldn't store it in the same table. If you have other fields you want to summarize then create a "summary" table and periodically summarize the fields and store the values there. ActiveRecord's other calculation methods might be of interest in that case.