何番煎じ？って感じですが、自分が知りたい情報を自分が見やすい形でまとめました。

結合系メソッドの比較表

joins
eager_load
preload
includes

補足: includesの「参照があればeager、なければpreload」というのは、関連テーブルを参照しているかどうかで、自動で使い分けてくれるということです。

User.includes(:books)
の場合はpreloadが使われます。

User.includes(:books).where(title: { "Ruby" })
場合はeager_loadを使われます。

ActiveRecordのクエリインターフェースと、発行されるSQLの対応表

今回使用したテーブル定義(簡易)

Userテーブル
- name
- age

Bookテーブル
- user_id
- title
- memo

Userが親、Bookが子
- User has_many Books
- Book belongs_to User

joins

戻り値サンプル

User.joins(:books)
=>
[#<User:0x000000010a074d00 id: 1, name: "user01", created_at: Fri, 12 May 2023 15:28:43.188967000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:06.987113000 UTC +00:00, age: 20>,
 #<User:0x000000010a074c60 id: 1, name: "user01", created_at: Fri, 12 May 2023 15:28:43.188967000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:06.987113000 UTC +00:00, age: 20>,
 #<User:0x000000010a074bc0 id: 2, name: "user02", created_at: Fri, 12 May 2023 15:28:49.759047000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:02.135818000 UTC +00:00, age: 20>]

User.joins(:books).where(books: { title: "Ruby" })
=>
[#<User:0x0000000107f5c190 id: 1, name: "user01", created_at: Fri, 12 May 2023 15:28:43.188967000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:06.987113000 UTC +00:00, age: 20>,
 #<User:0x00000001075f2960 id: 1, name: "user01", created_at: Fri, 12 May 2023 15:28:43.188967000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:06.987113000 UTC +00:00, age: 20>,
 #<User:0x00000001075f28c0 id: 2, name: "user02", created_at: Fri, 12 May 2023 15:28:49.759047000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:02.135818000 UTC +00:00, age: 20>]

eager_load

戻り値サンプル

User.eager_load(:books)
=>
[#<User:0x0000000108c38c38 id: 1, name: "user01", created_at: Fri, 12 May 2023 15:28:43.188967000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:06.987113000 UTC +00:00, age: 20>,
 #<User:0x000000010961f5a8 id: 2, name: "user02", created_at: Fri, 12 May 2023 15:28:49.759047000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:02.135818000 UTC +00:00, age: 20>,
 #<User:0x000000010961f328 id: 3, name: "user03", created_at: Fri, 12 May 2023 15:43:17.323247000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:17.323247000 UTC +00:00, age: 30>]

User.eager_load(:books).where(books: { title: "Ruby" })
=>
[#<User:0x00000001076a8530 id: 1, name: "user01", created_at: Fri, 12 May 2023 15:28:43.188967000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:06.987113000 UTC +00:00, age: 20>,
 #<User:0x000000010768db68 id: 2, name: "user02", created_at: Fri, 12 May 2023 15:28:49.759047000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:02.135818000 UTC +00:00, age: 20>]

preload

戻り値サンプル

User.preload(:books)
=>
[#<User:0x00000001082164a8 id: 1, name: "user01", created_at: Fri, 12 May 2023 15:28:43.188967000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:06.987113000 UTC +00:00, age: 20>,
 #<User:0x0000000108216408 id: 2, name: "user02", created_at: Fri, 12 May 2023 15:28:49.759047000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:02.135818000 UTC +00:00, age: 20>,
 #<User:0x0000000108216368 id: 3, name: "user03", created_at: Fri, 12 May 2023 15:43:17.323247000 UTC +00:00, updated_at: Fri, 12 May 2023 15:43:17.323247000 UTC +00:00, age: 30>]

User.preload(:books).where(books: { title: "Ruby" })
=>
<ActiveRecord::StatementInvalid: Mysql2::Error: Unknown column 'books.title' in 'where clause'>