【Ruby on Rails】「bullet」、「N+1問題」について

準備

まずは、モデルを作成します。

rails g model user name:string
rails g model post name:string

1 2	rails g model user name:string rails g model post name:string

マイグレーションファイルをそれぞれ作成します。

user

class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :name

      t.timestamps
    end
  end
end

class CreateUsers < ActiveRecord::Migration[6.0]

def change

create_table :users do |t|

t.string :name

t.timestamps

end

post

class CreatePosts < ActiveRecord::Migration[6.0]
  def change
    create_table :posts do |t|
      t.string :name
      t.references :user

      t.timestamps
    end
  end
end

class CreatePosts < ActiveRecord::Migration[6.0]

def change

create_table :posts do |t|

t.string :name

t.references :user

t.timestamps

end

modelファイルもそれぞれ設定します。

user

class User < ApplicationRecord
  has_many :posts
end

class User < ApplicationRecord

has_many :posts

end

post

class Post < ApplicationRecord
  belongs_to :user
end

class Post < ApplicationRecord

belongs_to :user

end

データ投入

「user：post = 1:多」になるようにデータを作成します。(rails cでRailsコンソール上で実行すると良いでしょう。)、ただ、userは1ですが、N+1問題を再現するために複数人作成します。

user(1)

user1 = User.create!(name:"太郎")
user2 = User.create!(name:"花子")
user3 = User.create!(name:"次郎")

user1 = User.create!(name:"太郎")

user2 = User.create!(name:"花子")

user3 = User.create!(name:"次郎")

post(多)

user1.posts.create!(name:"投稿1")
user1.posts.create!(name:"投稿2")
user2.posts.create!(name:"投稿1")
user2.posts.create!(name:"投稿2")
user2.posts.create!(name:"投稿3")
user3.posts.create!(name:"投稿1")
user3.posts.create!(name:"投稿2")
user3.posts.create!(name:"投稿3")

user1.posts.create!(name:"投稿1")

user1.posts.create!(name:"投稿2")

user2.posts.create!(name:"投稿1")

user2.posts.create!(name:"投稿2")

user2.posts.create!(name:"投稿3")

user3.posts.create!(name:"投稿1")

user3.posts.create!(name:"投稿2")

user3.posts.create!(name:"投稿3")

コントローラの作成

rails g controller users index

1	rails g controller users index

コントローラ側

class UsersController < ApplicationController
  def index
    @users = User.all
  end
end

class UsersController < ApplicationController

def index

@users = User.all

end

ビュー側

<% @users.each do |user| %>
  <%= user.name %> |
  <% user.posts.each do |post| %>
    <%= post.name %>
  <% end %>
  <br>
<% end %>

<% @users.each do |user| %>

<%= user.name %> |

<% user.posts.each do |post| %>

<%= post.name %>

<% end %>

<br>

<% end %>

url

http://localhost:3000/users

1	http://localhost:3000/users

上記URLにアクセスすると下記のような感じでselect文が大量に発行されるN+1問題が発生します。

bulletの導入

bulletは開発環境でしか基本使わないのでdevelopmentグループ配下に設定します。

group :development do
  gem 'bullet'
end

group :development do

gem 'bullet'

end

bundleインストールします。

bundle install

1	bundle install

以下の場所の設定ファイルにbulletで検知できるように設定します。

config/environments/development.rb

1	config/environments/development.rb

設定内容

config.after_initialize do
  Bullet.enable = true
  Bullet.alert = true
  Bullet.bullet_logger = true
  Bullet.console = true
  Bullet.rails_logger = true
end

config.after_initialize do

Bullet.enable = true

Bullet.alert = true

Bullet.bullet_logger = true

Bullet.console = true

Bullet.rails_logger = true

end

Railsサーバーを再起動します。

rails s

rails s

また、上記URLにアクセスすると下記のポップアップ表示がなされています。

ポップアップの指示通りコントローラのクエリに「includes(:posts)」を追加します。

@users = User.all.includes(:posts)

1	@users = User.all.includes(:posts)

再びページにアクセスすると今度はSQLが下記のようになりポップアップが出なくなっています。

  User Load (0.3ms)  SELECT "users".* FROM "users"
  ↳ app/views/users/index.html.erb:1
  Post Load (0.2ms)  SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN (?, ?, ?)  [["user_id", 1], ["user_id", 2], ["user_id", 3]]

User Load (0.3ms) SELECT "users".* FROM "users"

↳ app/views/users/index.html.erb:1

Post Load (0.2ms) SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN (?, ?, ?) [["user_id", 1], ["user_id", 2], ["user_id", 3]]

余分な場合もある。

余分にincludes等のN+1対策をしてしまっている場合は逆に下記のようなメッセージが出ます。

AVOID eager loading detected
Remove to your query: .includes([:モデル名])

1 2	AVOID eager loading detected Remove to your query: .includes([:モデル名])

その場合はincludes等を外しましょう。

N+1問題とは？

Railsでループ処理の都度SQLを発行してしまいパフォーマンスが低下する問題のことです。

具体的にはどんな動作か？

1:多のリレーションのテーブル同士で1に紐づいている多の情報を取得しようとした際に、最初に一括で1の方のテーブルのデータをSQLで取得した後に、他の多の方の関連データを1件1件ずつ合計N件になるまでSQLを発行し続けます。数件程度であれば問題ないのですがデータ量が増えると多大なパフォーマンスの損失に繋がりかねません。

具体例

具体的にはjoinsを利用していて結合したテーブルをループして関連先のテーブルのデータを参照しようとした場合に発生します。

projects = Project.joins(:category_projects)
  Project Load (7.5ms)  SELECT "projects".* FROM "projects" INNER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]>

irb(main):011:0> projects.each do |project| p project.category_projects end
  CategoryProject Load (0.8ms)  SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" = ? LIMIT ?  [["project_id", 1], ["LIMIT", 11]]
#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 1, category_id: 1, project_id: 1, created_at: "2020-12-19 09:56:20", updated_at: "2020-12-19 09:56:20">]>
  CategoryProject Load (0.5ms)  SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" = ? LIMIT ?  [["project_id", 2], ["LIMIT", 11]]
#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 2, category_id: 1, project_id: 2, created_at: "2020-12-19 09:56:28", updated_at: "2020-12-19 09:56:28">]>
=> [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]

projects = Project.joins(:category_projects)

Project Load (7.5ms) SELECT "projects".* FROM "projects" INNER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" LIMIT ? [["LIMIT", 11]]

=> #<ActiveRecord::Relation [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]>

irb(main):011:0> projects.each do |project| p project.category_projects end

CategoryProject Load (0.8ms) SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" = ? LIMIT ? [["project_id", 1], ["LIMIT", 11]]

#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 1, category_id: 1, project_id: 1, created_at: "2020-12-19 09:56:20", updated_at: "2020-12-19 09:56:20">]>

CategoryProject Load (0.5ms) SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" = ? LIMIT ? [["project_id", 2], ["LIMIT", 11]]

#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 2, category_id: 1, project_id: 2, created_at: "2020-12-19 09:56:28", updated_at: "2020-12-19 09:56:28">]>

=> [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]

上記例では「Project：Category_project」＝「1:多」になっている場合で関連先のテーブルを呼ぶたびにSELECT文が呼ばれていることがわかります。これはjoinsの特性としてメモリ上に関連先テーブル情報（CategoryProject）までキャッシュしないためです。

N+1問題への対策

下記のActiveRecordのメソッドを使うことにより比較的パフォーマンスを負荷を軽減できます。どちらも処理の前にassociationをキャッシュすることによりN回のSQL実行を1回にまとめることができます。

preloadメソッド
eager_loadメソッド

preloadメソッド

IN句を使ってSQLを一つにまとめます。あまりJOINしたくないでかいテーブル同士の結合の際はこちらを使うと良いでしょう。

irb(main):019:0> projects = Project.preload(:category_projects)
  Project Load (5.7ms)  SELECT "projects".* FROM "projects" LIMIT ?  [["LIMIT", 11]]
  CategoryProject Load (0.6ms)  SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" IN (?, ?)  [["project_id", 1], ["project_id", 2]]
=> #<ActiveRecord::Relation [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]>

irb(main):020:0> projects.each do |project| p project.category_projects end
  Project Load (0.8ms)  SELECT "projects".* FROM "projects"
  CategoryProject Load (0.4ms)  SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" IN (?, ?)  [["project_id", 1], ["project_id", 2]]
#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 1, category_id: 1, project_id: 1, created_at: "2020-12-19 09:56:20", updated_at: "2020-12-19 09:56:20">]>
#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 2, category_id: 1, project_id: 2, created_at: "2020-12-19 09:56:28", updated_at: "2020-12-19 09:56:28">]>
=> [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]

irb(main):019:0> projects = Project.preload(:category_projects)

Project Load (5.7ms) SELECT "projects".* FROM "projects" LIMIT ? [["LIMIT", 11]]

CategoryProject Load (0.6ms) SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" IN (?, ?) [["project_id", 1], ["project_id", 2]]

irb(main):020:0> projects.each do |project| p project.category_projects end

Project Load (0.8ms) SELECT "projects".* FROM "projects"

CategoryProject Load (0.4ms) SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" IN (?, ?) [["project_id", 1], ["project_id", 2]]

#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 1, category_id: 1, project_id: 1, created_at: "2020-12-19 09:56:20", updated_at: "2020-12-19 09:56:20">]>

#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 2, category_id: 1, project_id: 2, created_at: "2020-12-19 09:56:28", updated_at: "2020-12-19 09:56:28">]>

eager_loadの使い分けとしては、「1：多」でデータ量が多いテーブルの結合の際に使用すると良いでしょう。

whereが使えない。

相手先のテーブルの絞り込みをしようとして、whereを使うとエラーになります。

irb(main):030:0> projects = Project.preload(:category_projects).where(category_projects: { project_id:1} )
  Project Load (3.6ms)  SELECT "projects".* FROM "projects" WHERE "category_projects"."project_id" = ? LIMIT ?  [["project_id", 1], ["LIMIT", 11]]
Traceback (most recent call last):
ActiveRecord::StatementInvalid (SQLite3::SQLException: no such column: category_projects.project_id)

irb(main):030:0> projects = Project.preload(:category_projects).where(category_projects: { project_id:1} )

Project Load (3.6ms) SELECT "projects".* FROM "projects" WHERE "category_projects"."project_id" = ? LIMIT ? [["project_id", 1], ["LIMIT", 11]]

Traceback (most recent call last):

ActiveRecord::StatementInvalid (SQLite3::SQLException: no such column: category_projects.project_id)

includesメソッド

IN句を使ってSQLを一つにまとめます。状況に応じて、preloadかeager_loadが使い分けられます。それほど実務で影響があるわけではありませんが、理解を深められた方であれば意図的にpreloadかeager_loadかを選択するようにしましょう。

irb(main):023:0> projects = Project.includes(:category_projects)
  Project Load (1.6ms)  SELECT "projects".* FROM "projects" LIMIT ?  [["LIMIT", 11]]
  CategoryProject Load (2.4ms)  SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" IN (?, ?)  [["project_id", 1], ["project_id", 2]]
=> #<ActiveRecord::Relation [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]>

irb(main):024:0> projects.each do |project| p project.category_projects end
  Project Load (0.4ms)  SELECT "projects".* FROM "projects"
  CategoryProject Load (4.4ms)  SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" IN (?, ?)  [["project_id", 1], ["project_id", 2]]
#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 1, category_id: 1, project_id: 1, created_at: "2020-12-19 09:56:20", updated_at: "2020-12-19 09:56:20">]>
#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 2, category_id: 1, project_id: 2, created_at: "2020-12-19 09:56:28", updated_at: "2020-12-19 09:56:28">]>
=> [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]

irb(main):023:0> projects = Project.includes(:category_projects)

Project Load (1.6ms) SELECT "projects".* FROM "projects" LIMIT ? [["LIMIT", 11]]

CategoryProject Load (2.4ms) SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" IN (?, ?) [["project_id", 1], ["project_id", 2]]

irb(main):024:0> projects.each do |project| p project.category_projects end

Project Load (0.4ms) SELECT "projects".* FROM "projects"

CategoryProject Load (4.4ms) SELECT "category_projects".* FROM "category_projects" WHERE "category_projects"."project_id" IN (?, ?) [["project_id", 1], ["project_id", 2]]

#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 1, category_id: 1, project_id: 1, created_at: "2020-12-19 09:56:20", updated_at: "2020-12-19 09:56:20">]>

#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 2, category_id: 1, project_id: 2, created_at: "2020-12-19 09:56:28", updated_at: "2020-12-19 09:56:28">]>

eager_loadメソッド

LEFT OUTER JOINを使ってSQLを一つにまとめます。

irb(main):015:0> projects = Project.eager_load(:category_projects)
  SQL (0.5ms)  SELECT DISTINCT "projects"."id" FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" LIMIT ?  [["LIMIT", 11]]
  SQL (0.7ms)  SELECT "projects"."id" AS t0_r0, "projects"."title" AS t0_r1, "projects"."created_at" AS t0_r2, "projects"."updated_at" AS t0_r3, "category_projects"."id" AS t1_r0, "category_projects"."category_id" AS t1_r1, "category_projects"."project_id" AS t1_r2, "category_projects"."created_at" AS t1_r3, "category_projects"."updated_at" AS t1_r4 FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" WHERE "projects"."id" IN (?, ?)  [["id", 1], ["id", 2]]
↑関連先が「多」になっていると上のSQLでDISTINCTが発生してしまいコストが増える。

irb(main):016:0> projects.each do |project| p project.category_projects end
  SQL (0.8ms)  SELECT "projects"."id" AS t0_r0, "projects"."title" AS t0_r1, "projects"."created_at" AS t0_r2, "projects"."updated_at" AS t0_r3, "category_projects"."id" AS t1_r0, "category_projects"."category_id" AS t1_r1, "category_projects"."project_id" AS t1_r2, "category_projects"."created_at" AS t1_r3, "category_projects"."updated_at" AS t1_r4 FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id"
#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 1, category_id: 1, project_id: 1, created_at: "2020-12-19 09:56:20", updated_at: "2020-12-19 09:56:20">]>
#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 2, category_id: 1, project_id: 2, created_at: "2020-12-19 09:56:28", updated_at: "2020-12-19 09:56:28">]>
=> [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">, #<Project id: 2, title: "project2", created_at: "2020-12-19 09:54:40", updated_at: "2020-12-19 09:54:40">]

irb(main):015:0> projects = Project.eager_load(:category_projects)

SQL (0.5ms) SELECT DISTINCT "projects"."id" FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" LIMIT ? [["LIMIT", 11]]

SQL (0.7ms) SELECT "projects"."id" AS t0_r0, "projects"."title" AS t0_r1, "projects"."created_at" AS t0_r2, "projects"."updated_at" AS t0_r3, "category_projects"."id" AS t1_r0, "category_projects"."category_id" AS t1_r1, "category_projects"."project_id" AS t1_r2, "category_projects"."created_at" AS t1_r3, "category_projects"."updated_at" AS t1_r4 FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" WHERE "projects"."id" IN (?, ?) [["id", 1], ["id", 2]]

↑関連先が「多」になっていると上のSQLでDISTINCTが発生してしまいコストが増える。

irb(main):016:0> projects.each do |project| p project.category_projects end

SQL (0.8ms) SELECT "projects"."id" AS t0_r0, "projects"."title" AS t0_r1, "projects"."created_at" AS t0_r2, "projects"."updated_at" AS t0_r3, "category_projects"."id" AS t1_r0, "category_projects"."category_id" AS t1_r1, "category_projects"."project_id" AS t1_r2, "category_projects"."created_at" AS t1_r3, "category_projects"."updated_at" AS t1_r4 FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id"

#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 1, category_id: 1, project_id: 1, created_at: "2020-12-19 09:56:20", updated_at: "2020-12-19 09:56:20">]>

#<ActiveRecord::Associations::CollectionProxy [#<CategoryProject id: 2, category_id: 1, project_id: 2, created_at: "2020-12-19 09:56:28", updated_at: "2020-12-19 09:56:28">]>

joinsと同じように記述していますが、ループを回す際のSQLの実行が1回で済んでいるのでSQL発行回数は最小に抑えることができます。ただ、相手が関連先テーブルがNの場合は最初に結合する際に、DISTINCTが発生するのでデータ量が少なく、「１：１」の関連の場合のみ使用すると良いでしょう。

whereも使える。

eager_loadなら相手先のテーブルで絞り込んだ結果を返せます。

irb(main):029:0> projects = Project.eager_load(:category_projects).where(category_projects: { project_id:1} )
  SQL (1.7ms)  SELECT DISTINCT "projects"."id" FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" WHERE "category_projects"."project_id" = ? LIMIT ?  [["project_id", 1], ["LIMIT", 11]]
  SQL (174.9ms)  SELECT "projects"."id" AS t0_r0, "projects"."title" AS t0_r1, "projects"."created_at" AS t0_r2, "projects"."updated_at" AS t0_r3, "category_projects"."id" AS t1_r0, "category_projects"."category_id" AS t1_r1, "category_projects"."project_id" AS t1_r2, "category_projects"."created_at" AS t1_r3, "category_projects"."updated_at" AS t1_r4 FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" WHERE "category_projects"."project_id" = ? AND "projects"."id" = ?  [["project_id", 1], ["id", 1]]
=> #<ActiveRecord::Relation [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">]>

irb(main):029:0> projects = Project.eager_load(:category_projects).where(category_projects: { project_id:1} )

SQL (1.7ms) SELECT DISTINCT "projects"."id" FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" WHERE "category_projects"."project_id" = ? LIMIT ? [["project_id", 1], ["LIMIT", 11]]

SQL (174.9ms) SELECT "projects"."id" AS t0_r0, "projects"."title" AS t0_r1, "projects"."created_at" AS t0_r2, "projects"."updated_at" AS t0_r3, "category_projects"."id" AS t1_r0, "category_projects"."category_id" AS t1_r1, "category_projects"."project_id" AS t1_r2, "category_projects"."created_at" AS t1_r3, "category_projects"."updated_at" AS t1_r4 FROM "projects" LEFT OUTER JOIN "category_projects" ON "category_projects"."project_id" = "projects"."id" WHERE "category_projects"."project_id" = ? AND "projects"."id" = ? [["project_id", 1], ["id", 1]]

=> #<ActiveRecord::Relation [#<Project id: 1, title: "project1", created_at: "2020-12-19 09:54:11", updated_at: "2020-12-19 09:54:11">]>

includesメソッド

例えば、Userテーブルに1：多のリレーションがあるpcテーブルがある場合に下記のように記述する場合

User.pc

User.pc

上記のように記述するとUserごとにslect文を発行して1つずつpcを取得する動作になってしまいます。

User.includes(:pc)

1	User.includes(:pc)

includesを使えばIN句を使ってユーザーに紐づくpcを一括して取得することができます。

【Ruby on Rails】「bullet」、「N+1問題」について

準備

user

post

user

post

データ投入

user(1)

post(多)

コントローラの作成

コントローラ側

ビュー側

url

bulletの導入

設定内容

余分な場合もある。

N+1問題とは？

具体的にはどんな動作か？

具体例

N+1問題への対策

preloadメソッド

whereが使えない。

includesメソッド

eager_loadメソッド

whereも使える。

includesメソッド

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