多：多のアソシエーションでN+1問題を起こさないための手段

前提

ブログとカテゴリがn:nの関係

app/models/blog.rb

class Blog < ApplicationRecord
  has_many :blogs_categories
  has_many :categories, through: :blogs_categories, source: :category
end

app/models/category.rb

class Category < ApplicationRecord
  has_many :blogs_categories
  has_many :blogs, through: :blogs_categories, source: :blog
end

app/models/blogs_category.rb

class BlogsCategory < ApplicationRecord
  belongs_to :blog
  belongs_to :category
end

ちなみに中間テーブルの名前は 複数_複数 とするのがRailsのルール。
今回なら blogs_categoriesテーブル 。
ちなみにモデル名は 複数_単数 とするみたい。
今回なら ファイル名が blogs_category.rb クラス名が BlogsCategory 。
ややこしや。。

includesのみ

アソシエーションごとにキャッシュ
今回は以下３つのテーブルがあるので

• blogsテーブル
• blogs_categoriesテーブル
• categoriesテーブル

クエリは３回

app/controllers/blogs_controller.rb

class BlogsController < ApplicationController
  def index
    @blogs = Blog.includes(:categories)
  end

  # 省略
end

Blog Load (0.7ms)  SELECT "blogs".* FROM "blogs"

BlogsCategory Load (0.9ms)  SELECT "blogs_categories".* FROM "blogs_categories" WHERE "blogs_categories"."blog_id" IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

Category Load (0.3ms)  SELECT "categories".* FROM "categories" WHERE "categories"."id" IN (1, 2, 3, 4, 5)

includes + references

LEFT OUTER JOINで１回のクエリでアソシエーションをキャッシュ

app/controllers/blogs_controller.rb

class BlogsController < ApplicationController
  def index
    @blogs = Blog.includes(:categories).references(:all)
  end

  # 省略
end

SELECT "blogs"."id" AS t0_r0, 
        "blogs"."title" AS t0_r1, 
        "blogs"."content" AS t0_r2,
        "blogs"."deleted_at" AS t0_r4, 
        "blogs"."created_at" AS t0_r5, 
        "blogs"."updated_at" AS t0_r6, 
        "blogs"."publish_at" AS t0_r7, 
        "blogs"."status" AS t0_r9, 
        "categories"."id" AS t1_r0, 
        "categories"."name" AS t1_r1, 
        "categories"."created_at" AS t1_r3,
        "categories"."updated_at" AS t1_r4,
FROM "blogs" 
LEFT OUTER JOIN "blogs_categories" 
        ON "blogs_categories"."blog_id" = "blogs"."id" 
LEFT OUTER JOIN "categories" 
        ON "categories"."id" = "blogs_categories"."category_id"