在 Rails 中使用范围急切加载

Question

我发现了许多标题相似的问题，但没有一个可以解决我的问题。

我有一个模型Program，其中有很多Videos：

class Program < ActiveRecord::Base
  has_many :videos
  ...
end

那么我在Video中有范围：

class Video < ActiveRecord::Base
  belongs_to :program

  scope :trailer, -> { where(video_type: 0) }
  ...
end

首先，当我有一个程序列表并想访问视频时，我没有使用include方法的N+1个程序：

> @programs.includes(:videos).map { |p| p.videos.size }
  Program Load (0.6ms)  SELECT  "programs".* FROM "programs"  ORDER BY "programs"."id" ASC LIMIT 10
  Video Load (0.5ms)  SELECT "videos".* FROM "videos" WHERE "videos"."program_id" IN (8, 9, 10, 11, 12, 13, 14, 15, 16, 17)

但是，当我尝试获取范围时，它会再次触及数据库：

> @programs.includes(:videos).map { |p| p.videos.trailer }
  Program Load (0.6ms)  SELECT  "programs".* FROM "programs"  ORDER BY "programs"."id" ASC LIMIT 10
  Video Load (0.5ms)  SELECT "videos".* FROM "videos" WHERE "videos"."program_id" IN (8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
  Video Load (0.4ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 8], ["video_type", 0]]
  Video Load (0.4ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 9], ["video_type", 0]]
  Video Load (12.4ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 10], ["video_type", 0]]
  Video Load (0.3ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 11], ["video_type", 0]]
  Video Load (0.3ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 12], ["video_type", 0]]
  Video Load (0.3ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 13], ["video_type", 0]]
  Video Load (0.3ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 14], ["video_type", 0]]
  Video Load (0.3ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 15], ["video_type", 0]]
  Video Load (0.4ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 16], ["video_type", 0]]
  Video Load (0.4ms)  SELECT  "videos".* FROM "videos" WHERE "videos"."program_id" = $1 AND "videos"."video_type" = $2  ORDER BY "videos"."id" ASC LIMIT 1  [["program_id", 17], ["video_type", 0]]

您可以看到它会多次加载 DB，从而导致性能不佳。

#<Benchmark::Tms:0x007f95faa8fab0 @label="", @real=0.02663199999369681, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.019999999999999574, @total=0.019999999999999574>

---

我能想到的一个解决方案是将视频转换为数组并搜索数组：

> @programs.includes(:videos).map { |program| program.videos.to_ary.select { |v| v.video_type == 0 } }
  Program Load (0.5ms)  SELECT "programs".* FROM "programs" WHERE "programs"."id" IN (8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
  Video Load (0.4ms)  SELECT "videos".* FROM "videos" WHERE "videos"."program_id" IN (17, 16, 13, 12, 11, 9, 8, 15, 14, 10)

性能更好，但代码复杂。

#<Benchmark::Tms:0x007f95faac8720 @label="", @real=0.006901999993715435, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.010000000000000675, @total=0.010000000000000675>

---

我能想到的另一个解决方案是在Program 中为范围添加一个新的has_many：

class Program < ActiveRecord::Base
  has_many :videos
  has_many :trailer_videos, -> { where(video_type: 0) }, class: 'Video'
  ...
end

然后如果我includes 并直接调用新关系，它也会立即加载。

> @programs.includes(:trailer_videos).map { |program| program.trailer_videos }
  Program Load (0.5ms)  SELECT "programs".* FROM "programs" WHERE "programs"."id" IN (8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
  Video Load (0.3ms)  SELECT "videos".* FROM "videos" WHERE "videos"."video_type" = $1 AND "videos"."program_id" IN (17, 16, 13, 12, 11, 9, 8, 15, 14, 10)  [["video_type", 0]]

基准如下，速度超快：

#<Benchmark::Tms:0x007f95fdea96c0 @label="", @real=0.004801000002771616, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.009999999999999787, @total=0.009999999999999787>

但是，这样一来，Program 模型就会变得如此沉重。因为对于Video中的每个作用域，我需要在Program中添加相关关联。

---

因此，我正在寻找更好的解决方案，它将范围逻辑保留在Video 内，但不会出现 N+1 问题。

干杯

Answer 1

正如我所说，IMO 您添加has_many :trailer_videos, -> { where(video_type: 0) }, class: 'Video' 的方法是解决您的问题的简单且最佳的方法。我认为向模型添加更多此类关联没有任何缺点。

Answer 2

一种解决方案是使用merge 和eager_load：

@programs.eager_load(:videos).merge(Video.trailer).map { |p| p.videos.size }

它只产生一个查询。

Answer 3

在 Rails 中 Associations 有一个可选的范围参数，该参数接受应用于 Relation 的 lambda（参见 https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many-label-Scopes）

所以你可以把你的模型写成：

# app/models/video.rb
class Video < ActiveRecord::Base
  belongs_to :program
  scope :trailer, -> { where(video_type: 0) }
  ...
end

# app/models/program.rb
class Program < ActiveRecord::Base
  has_many :videos
  has_many :trailer_videos, -> { trailer }, class: 'Video'
  ...
end

这样您可以将范围的定义保留在Video 中，并从Program 重用它。

Answer 4

如果程序员知道视频类型，您可以使用ActiveRecord::Enum 和一些简单的元编程以编程方式为枚举中的每个可能值创建关联。

class Video < ActiveRecord::Base
  enum video_type: [:trailer, :promo, :foo, :bar]
end

class Program < ActiveRecord::Base
  # this creates trailer_videos etc assocations
  Video.video_types.each do |key, int| 
    # eval is needed since we need to dynamically create 
    # the lamba for each type
    has_many "#{key}_videos".to_sym, eval "->{ Video.send(#{key}) }"
  end
end