Question

Rails 3.2。我有以下内容：

# city.rb
class City < ActiveRecord::Base
  has_many :zones, :dependent => :destroy
end

# zone.rb
class Zone < ActiveRecord::Base
  belongs_to :city
  has_many :zone_shops, :dependent => :destroy
  has_many :shops, :through => :zone_shops
end

# zone_shop.rb
class ZoneShop < ActiveRecord::Base
  belongs_to :zone
  belongs_to :shop
end

# shop.rb
class Shop < ActiveRecord::Base
end

# cities_controller.rb
def show
  @trip = City.find(params[:id], :include => [:user, :zones => [:shops]])
  @zones = @trip.zones.order("position")

  # List out all shops for the trip
  shops_list_array = []
  @zones.each do |zone, i|
    zone.shops.each do |shop|
      shops_list_array << shop.name
    end
  end
  @shops_list = shops_list_array.join(', ')
end

# development.log
  City Load (0.3ms)  SELECT `cities`.* FROM `cities` WHERE `cities`.`id` = 1 LIMIT 1
  Zone Load (0.3ms)  SELECT `zones`.* FROM `zones` WHERE `zones`.`trip_id` IN (1) ORDER BY position asc
  ZoneShop Load (0.3ms)  SELECT `zone_shops`.* FROM `zone_shops` WHERE `zone_shops`.`zone_id` IN (26, 23, 22) ORDER BY position asc
  Shop Load (0.5ms)  SELECT `shops`.* FROM `shops` WHERE `shops`.`id` IN (8, 7, 1, 9)
  Zone Load (0.5ms)  SELECT `zones`.* FROM `zones` WHERE `zones`.`trip_id` = 1 ORDER BY position asc, position
  Shop Load (0.5ms)  SELECT `shops`.* FROM `shops` INNER JOIN `zone_shops` ON `shops`.`id` = `zone_shops`.`spot_id` WHERE `zone_shops`.`zone_id` = 26
  Shop Load (0.6ms)  SELECT `shops`.* FROM `shops` INNER JOIN `zone_shops` ON `shops`.`id` = `zone_shops`.`spot_id` WHERE `zone_shops`.`zone_id` = 23
  Shop Load (0.4ms)  SELECT `shops`.* FROM `shops` INNER JOIN `zone_shops` ON `shops`.`id` = `zone_shops`.`spot_id` WHERE `zone_shops`.`zone_id` = 22

请注意，在我的log中，商店ID为26,23,22的最后3行是多余的。我应该如何重写cities_controller.rb以减少对系统的查询？

非常感谢。

Answer 1

@zones = @trip.zones.includes(:shops).order("position")

这会急切加载商店协会，并且应该消除由zone.shops.each

引起的n + 1查询问题

有关更多信息，请查看Ruby on Rails Guide section 12 on Eager Loading associations，该链接也由@Benjamin M链接

Answer 2

我建议这个

zone.shops.each do |shop|
  shops_list_array << shop.name
end

生成日志的最后3行。这意味着：您当前在数据库中只有一个zone。如果你在那里放置更多区域，你将在日志中获得更多Zone Load个条目。

问题显然是Rails'each方法，它会触发延迟加载：

@zones.each do |zone, i|
  ...

解决方案取决于您的需求，但我建议您阅读有关Rails的急切加载功能的所有内容。（确切地说是你的问题：each事情）。在这里查找：http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations

这很简单，简单明了：）

优化Rails中的热切加载

2 个答案: