Question

我有这些模特：

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

球队拥有运动类型（“足球”），球员拥有主场状态（“CA”）。

我想要一份来自加利福尼亚的每个足球运动员的名字列表。

SQL看起来像

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足球运动员fb_players = Sport.find_all_by_sport_nane("Football")，然后遍历所有fb_players.players并查看谁来自加利福尼亚，但这感觉比单个SQL语句慢

由于

Answer 1

试试：

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

回答你的评论：

执行.map(&:firstName)就像做.map{ |player| player.firstName }一样。基本上，&正在将Proc转换为块以传递给map方法。但:firstName不是块？！对。但是ruby会自动在符号上调用to_proc方法。

to_proc方法定义如下：

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

Answer 2

尝试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

Rails - 更聪明的方式加入？

2 个答案: