gistfile1.txt

```
# コメントは2階層で、親→子はreplies、子→親はparentとして参照する
class Comment < ApplicationRecord
  belongs_to :article

  belongs_to :parent, class_name: :Comment, optional: true
  has_many :replies, class_name: :Comment, foreign_key: :parent_id
end
```

```scope :with_all_comments, -> {
    eager_load(comments: :replies).where(comments: {parent_id: nil})
  }
```
    
```
# articles.comments はwith_all_commentsによって得られた、parent_idが空の(親階層の)comments
    replies = Comment.find_by_sql [
      # 各コメントに対して最新の返信1件のみを取得したい
      "
        select * from comments where id in (
          select max(id) from comments where parent_id in (?) group by parent_id
        )
      ",
      articles.flat_map(&:comment_ids)
    ]
    articles.each do |a|
      a.comments.each do |c|
        # TODO: use group_by
        c.replies = replies.select do |r|
          r.parent_id == c.id
        end
      end
    end
```