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Rails关联-改变值的问题,以及太多的缓存!

associations caching ruby-on-rails

0

Chowlett · 技术社区 · 16 年前

Player 模型,它有一个 actions 整数列;和 Card 模型。玩家可以玩自己拥有的一张牌,这需要一个动作;一张特定的牌在玩的时候会给予两个动作。

class Player < ActiveRecord::Base
  has_many :cards

  def play_card(card)
    raise "Not yours!" unless cards.include? card
    self.actions -= 1
    card.play
    save!
  end
end

class Card < ActiveRecord::Base
  belongs_to :player

  def play
    player.actions += 2
  end
end

... 那么 Player#play_card 行动 按1。我发现对同一个对象进行这两个更改的唯一方法是定义如下函数:

class Player < ActiveRecord::Base
  has_many :cards

  def play_card(card)
    raise "Not yours!" unless cards.include? card
    self.actions -= 1

    // Stick that change in the Database
    save!

    card.play
  end
end

class Card < ActiveRecord::Base
  belongs_to :player

  def play
    // Force reload of the player object
    player(true).actions += 2

    // And save again
    player.save!
  end
end

但这会把一个数据库的写操作变成两个写操作和一个读操作!肯定有更好的办法。我错过了什么?

1 回复 | 直到 16 年前

1

0

hellvinz 16 年前

在您的代码的第一个版本中,您正在加载表播放器的同一行,但是当您希望rails足够聪明,能够识别它已经在内存中加载了这一行时,rails并不是这样工作的。所以当你在player上发出a+=2时,它会在另一个实例上执行he+=2,而不是在你已经执行过的实例上执行-=1。

我设置了一个小示例来说明同一行的实例太多:

ruby-1.8.7-p174 > p_instance_1 = Player.first
 => #<Player id: 1, actions: -1, created_at: "2010-10-13 17:07:22", updated_at: "2010-10-13 17:11:00"> 
ruby-1.8.7-p174 > c = Card.first
 => #<Card id: 1, player_id: 1, created_at: "2010-10-13 17:07:28", updated_at: "2010-10-13 17:07:28"> 
ruby-1.8.7-p174 > p_instance_2 = c.player
 => #<Player id: 1, actions: -1, created_at: "2010-10-13 17:07:22", updated_at: "2010-10-13 17:11:00"> 
ruby-1.8.7-p174 > p_instance_1.object_id
 => 2158703080 
ruby-1.8.7-p174 > p_instance_2.object_id
 => 2156926840 
ruby-1.8.7-p174 > p_instance_1.actions += 1
 => 0 
ruby-1.8.7-p174 > p_instance_2.actions += 1
 => 0

更新

你可以尝试欺骗rails,让它一直使用player的同一个实例。这有点难看,但很管用。

class Player < ActiveRecord::Base
  has_many :cards

  def play_card(card)
    raise "Not yours!" unless cards.include? card
    new_self = card.player
    card.play
    new_self.actions -= 1
    new_self.save!
  end
end

class Card < ActiveRecord::Base
  belongs_to :player

  def play
    player.actions += 2
  end
end

ruby-1.8.7-p174 > p = Player.first
 => #<Player id: 1, actions: 0, created_at: "2010-10-14 13:33:51", updated_at: "2010-10-14 13:33:51"> 
ruby-1.8.7-p174 > p.play_card(Card.first)
 => true 
ruby-1.8.7-p174 > p
 => #<Player id: 1, actions: 0, created_at: "2010-10-14 13:33:51", updated_at: "2010-10-14 13:33:51"> 
ruby-1.8.7-p174 > p.reload
 => #<Player id: 1, actions: 1, created_at: "2010-10-14 13:33:51", updated_at: "2010-10-14 13:34:40">

你在player中有正确数量的动作,而在logs卡中只加载一次:

  Player Load (0.5ms)   SELECT * FROM "players" LIMIT 1
  Card Load (0.2ms)   SELECT * FROM "cards" LIMIT 1
  Card Load (0.2ms)   SELECT "cards".id FROM "cards" WHERE ("cards"."id" = 1) AND ("cards".player_id = 1) LIMIT 1
  Player Load (0.1ms)   SELECT * FROM "players" WHERE ("players"."id" = 1) 
  Player Update (0.6ms)   UPDATE "players" SET "updated_at" = '2010-10-14 13:34:40', "actions" = 1 WHERE "id" = 1

总而言之,我想说你的代码设计出了问题。如果我理解得很好,您希望的是表行的每个AR实例都是ObjectSpace中的同一个对象,但是我猜如果rails是这样构建的,那么它会引入一些奇怪的行为,您可以在验证和其他钩子中处理更改的半备份对象。