ruby-on-rails - 有没有办法急切加载多态关联的关联？

Question

艺术家有很多事件(基本上是用户之间交互的缓存):

class Activity < ActiveRecord::Base

  belongs_to :receiver, :class_name => 'Artist', :foreign_key => :receiver_id #owns the stuff done "TO" him
  belongs_to :link, :polymorphic => true
  belongs_to :creator, :class_name => 'Artist', :foreign_key => :creator_id #person who initiated the activity

end

例如:

  Activity.create(:receiver_id => author_id, :creator_id => artist_id, :link_id => id, :link_type => 'ArtRating') 

我想为每个艺术家创建一个事件流页面，由不同类型的事件列表、ArtRatings(喜欢、不喜欢)、收藏、关注等组成。

Controller 如下所示:

class ActivityStreamController < ApplicationController
  def index
    @activities = @artist.activities.includes([:link,:creator,:receiver]).order("id DESC").limit(30)
  end
end

db 调用正确地急切地加载多态链接对象:

  SELECT "activities".* FROM "activities" WHERE (("activities"."receiver_id" = 6 OR "activities"."creator_id" = 6)) ORDER BY id DESC LIMIT 30
  ArtRating Load (0.5ms)  SELECT "art_ratings".* FROM "art_ratings" WHERE "art_ratings"."id" IN (137, 136, 133, 130, 126, 125, 114, 104, 103, 95, 85, 80, 73, 64)
  SELECT "follows".* FROM "follows" WHERE "follows"."id" IN (14, 10)
  SELECT "favorites".* FROM "favorites" WHERE "favorites"."id" IN (25, 16, 14)

但是当我显示每个ArtRating时，我还需要引用帖子标题，它属于一个帖子。在 View 中，如果我这样做:

activity.link.post

它对每个 art_rating 的帖子进行单独的数据库调用。有没有办法也可以急切地加载帖子？

问题更新:

如果无法使用“包含”语法实现帖子的预加载，有没有办法自己手动执行预加载查询并将其注入(inject)@activities 对象？

我在数据库日志中看到:

SELECT "art_ratings".* FROM "art_ratings" WHERE "art_ratings"."id" IN (137, 136, 133, 130, 126, 125, 114, 104, 103, 95, 85, 80, 73, 64)

有没有办法可以从 @activities 对象访问这个 id 列表？如果是这样，我可以做 2 个额外的查询，1 个来获取该列表中的 art_ratings.post_id(s)，另一个来选择这些 post_ids 列表中的所有帖子。然后以某种方式将“发布”结果注入(inject)@activities，以便在我遍历集合时它可以作为 activity.link.post 使用。可能的？

Answer 1

TL;DR 我的解决方案使 artist.created_activities.includes(:link)急切地加载你想要的一切

这是我的第一次尝试:https://github.com/JohnAmican/music

几点注意事项:

我依赖 default_scope ，所以这不是最优的。

看起来您正在使用 STI。我的解决方案没有。这意味着您不能简单地调用activities关于艺术家；您必须引用 created_activities或 received_activities .可能有办法解决这个问题。如果我发现任何东西，我会更新。

我改变了一些名字，因为否则它会让我感到困惑。

如果您进入控制台并执行 created_activities.includes(:link) ，适当的东西被急切地加载:

irb(main):018:0> artist.created_activities.includes(:link)
  Activity Load (0.2ms)  SELECT "activities".* FROM "activities" WHERE "activities"."creator_id" = ?  [["creator_id", 1]]
  Rating Load (0.3ms)  SELECT "ratings".* FROM "ratings" WHERE "ratings"."id" IN (1)
  RatingExplanation Load (0.3ms)  SELECT "rating_explanations".* FROM "rating_explanations" WHERE "rating_explanations"."rating_id" IN (1)
  Following Load (0.3ms)  SELECT "followings".* FROM "followings" WHERE "followings"."id" IN (1)
  Favorite Load (0.2ms)  SELECT "favorites".* FROM "favorites" WHERE "favorites"."id" IN (1)
=> #<ActiveRecord::Relation [#<Activity id: 1, receiver_id: 2, creator_id: 1, link_id: 1, link_type: "Rating", created_at: "2013-10-31 02:36:27", updated_at: "2013-10-31 02:36:27">, #<Activity id: 2, receiver_id: 2, creator_id: 1, link_id: 1, link_type: "Following", created_at: "2013-10-31 02:36:41", updated_at: "2013-10-31 02:36:41">, #<Activity id: 3, receiver_id: 2, creator_id: 1, link_id: 1, link_type: "Favorite", created_at: "2013-10-31 02:37:04", updated_at: "2013-10-31 02:37:04">]>

至少，这证明了 Rails 有能力做到这一点。规避 default_scope告诉 Rails 你想做什么似乎是一个问题，而不是技术限制。

更新:

事实证明，当您将范围 block 传递给关联并调用该关联时， self在该 block 内指的是关系。因此，您可以反射(reflection)并采取适当的行动:

class Activity < ActiveRecord::Base
  belongs_to :creator,  class_name: 'Artist', foreign_key: :creator_id,  inverse_of: :created_activities
  belongs_to :receiver, class_name: 'Artist', foreign_key: :receiver_id, inverse_of: :received_activities
  belongs_to :link, -> { self.reflections[:activity].active_record == Rating ? includes(:rating_explanation) : scoped }, polymorphic: true
end

我更新了我的代码以反射(reflect)(哈哈)这一点。

这可以清理。例如，也许您并不总是想要预先加载 rating_explanations访问事件链接时。有很多方法可以解决这个问题。如果你愿意，我可以发一个。

但是，我认为这表明最重要的是 在关联的范围 block 内，您可以访问 ActiveRecord::Relation正在 build 。这将允许您有条件地对其进行操作。