Rails:意外的急切加载具有许多连接模型

Question

使用Rails 5.0和Postgresql的开发环境。我们有3个通过关系实现has_many的机型：

class Account < ApplicationRecord
  has_many :account_campaigns
  has_many :campaigns, through: :account_campaigns

  def self.list_campaigns
    self.joins(:account_campaigns).select('accounts.id, array_agg(campaign_id) AS campaign_ids').group('accounts.id')
  end
end

class Campaign < ApplicationRecord
  has_many :account_campaigns
  has_many :accounts, through: :account_campaigns
end

class AccountCampaign < ApplicationRecord
  belongs_to :account
  belongs_to :campaign
end

以及一个控制器，该控制器具有一个动作，用于获取以json格式作为api的活动列表的帐户：

class DashboardController < ApplicationController
  def index
    render json: Account.list_campaigns
  end
end

结果看起来不错，但有额外的查询来获得我们意想不到的活动：

Started GET "/" for 127.0.0.1 at 2017-03-22 12:06:04 +0700
Processing by DashboardController#index as HTML
  User Load (0.5ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT $2  [["id", 1], ["LIMIT", 1]]
  Account Load (0.5ms)  SELECT accounts.id, array_agg(campaign_id) AS campaign_ids FROM "accounts" INNER JOIN "account_campaigns" ON "account_campaigns"."account_id" = "accounts"."id" GROUP BY accounts.id

"### extra queries here ###"
   (0.5ms)  SELECT "campaigns".id FROM "campaigns" INNER JOIN "account_campaigns" ON "campaigns"."id" = "account_campaigns"."campaign_id" WHERE "account_campaigns"."account_id" = $1  [["account_id", 1]]
   (0.4ms)  SELECT "campaigns".id FROM "campaigns" INNER JOIN "account_campaigns" ON "campaigns"."id" = "account_campaigns"."campaign_id" WHERE "account_campaigns"."account_id" = $1  [["account_id", 2]]
Completed 200 OK in 100ms (Views: 57.3ms | ActiveRecord: 18.8ms)

为什么要执行这些额外的查询？在这种情况下，如何关闭预加载以优化性能，因为会有大量的数据？

Answer 1

我们发现急切加载是由控制器中的render json触发的。并且select语句中标记的列campaign_ids被用来急切加载。为了解决这个问题，我们替换查询：

self.joins(:account_campaigns).select('accounts.id, array_agg(campaign_id) AS campaign_ids').group('accounts.id')

出自：

self.joins(:account_campaigns).select('accounts.id, array_agg(campaign_id) AS cids').group('accounts.id')

唯一的区别是将标签campaign_ids更改为cids (或与campaign_ids不同的任何内容)，以避免运行时的急切负载。