Rails -更聪明的连接方式？

Question

我有这些模型：

class Player < ActiveRecord::Base
  has_many :players_to_teams
  has_many :teams, through: :players_to_teams
end

class Team < ActiveRecord::Base
  has_many :players_to_teams
  has_many :players, through: :players_to_teams
end

class PlayersToTeam < ActiveRecord::Base
  belongs_to :player
  belongs_to :team
end

球队有运动类型("Football")，球员有主场状态("CA")。

我要一份加州所有橄榄球运动员的名字名单。

SQL将如下所示

SELECT p.FirstName 
FROM players AS p
INNER JOIN players_to_teams AS ptt
ON p.id = ptt.player_id 
INNER JOIN teams AS t 
ON t.id = ptt.team_id 
WHERE t.sport_name = "Football" AND p.home_state = "CA"

我唯一能想到的就是让所有的橄榄球运动员fb_players = Sport.find_all_by_sport_nane("Football")，然后遍历所有的fb_players.players，看看谁来自加利福尼亚，但这感觉比一条SQL语句要慢得多。

谢谢

Answer 1

试一试：

Player.select("firstName").joins(:teams).where(:"teams.sport_name" => "Football", :home_state => "CA").map(&:firstName)

要回答您的评论：

做.map(&:firstName)就像做.map{ |player| player.firstName }一样。基本上，&将一个Proc转换为一个要传递给map方法的块。但是:firstName不是一个区块？！正确的。但是拼音会自动在符号上调用to_proc方法。

to_proc方法的定义如下：

def to_proc
    proc { |obj, *args| obj.send(self, *args) }
end

Answer 2

试试has_and_belongs_to_many

class Player < ActiveRecord::Base
  has_and_belongs_to_many :teams, :join_table = "players_to_teams"
end

class Team < ActiveRecord::Base
  has_and_belongs_to_many :players, :join_table = "players_to_teams"
end

http://jonathanhui.com/ruby-rails-3-model-many-many-association