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INNER JOIN じゃないと不味いが joins だと N + 1 問題が発生してしまう場合にどうするかを調べた。 結論から言うと joins + preload または joins + eager_load を使うと INNER JOIN を使って結合した上で関連オブジェクトも即時フェッチしてくれる。 試したのは Rails 4.1.6。 joins + preload INNER JOIN で結合 クエリ複数回(指定した関連オブジェクトの分) 即時フェッチ > Post.joins(:comments).preload(:comments) Post Load (0.3ms) SELECT "posts".* FROM "posts" INNER JOIN "comments" ON "comments"."post_id" = "posts"."id" Comment Load