Rails 3 - 如何通过 order by 从模型中获取行号

我正在制作一个小应用程序，其中包含排行榜概念。基本上，该模型只是一个player_name 和一个current_score。

我想做的是获取特定玩家的排名，并且考虑到新的分数一直在出现，它需要是动态的。显然，我可以使用 order by 子句进行正常查找，然后我必须循环遍历每个记录。当我有 100,000 行时，效率不是很高。

关于我应该采取什么方法有什么建议吗？

这是表的迁移：

class CreateScores < ActiveRecord::Migration
  def self.up
    create_table :scores do |t|
      t.string  :player_name
      t.integer :current_score

      t.timestamps
    end
  end

  def self.down
    drop_table :scores
  end
end

编辑 作为示例，我有以下内容：

Score.select('player_name, current_score').limit(20)
=> [#<Score player_name: "Keith Hughes", current_score: 9>, #<Score player_name: "Diane Chapman", current_score: 8>, #<Score player_name: "Helen Dixon", current_score: 4>, #<Score player_name: "Donald Lynch", current_score: 9>, #<Score player_name: "Shawn Snyder", current_score: 2>, #<Score player_name: "Nancy Palmer", current_score: 9>, #<Score player_name: "Janet Arnold", current_score: 1>, #<Score player_name: "Sharon Torres", current_score: 9>, #<Score player_name: "Keith Ortiz", current_score: 5>, #<Score player_name: "Judith Day", current_score: 3>, #<Score player_name: "Gregory Watson", current_score: 7>, #<Score player_name: "Jeremy Welch", current_score: 3>, #<Score player_name: "Sharon Oliver", current_score: 7>, #<Score player_name: "Donald Lewis", current_score: 7>, #<Score player_name: "Timothy Frazier", current_score: 7>, #<Score player_name: "John Richards", current_score: 1>, #<Score player_name: "Carolyn White", current_score: 4>, #<Score player_name: "Ronald Smith", current_score: 2>, #<Score player_name: "Emily Freeman", current_score: 9>, #<Score player_name: "Gregory Wright", current_score: 2>]

我怎样才能计算出“唐纳德·刘易斯”的排名？

您可以计算具有较高 current_score 的记录数。您仍然需要按记录执行此操作（或在 sql 中执行子查询）。

class Score < ActiveRecord::Base

  def ranking
    Score.count(:conditions => ['current_score > ?', self.current_score])
  end

end