Nested comments in ROM

Architecture Design
1

Hi everyone!
Previously I only worked with an Active Record. And I want to create a comment system for news.
In AR I could use polymorphic associations for this. But this is not there.

This is my relations:

app/relations/comments_relation.rb

class CommentsRelation < ROM::Relation[:sql]
gateway :default

  schema(:comments, infer: true) do
    associations do
      belongs_to :news
      belongs_to :user
      has_many   :comments, foreign_key: :parent_comments_id
      belongs_to :comments, as: :parent
    end
  end
end

app/relations/news_relation.rb

class NewsRelation < ROM::Relation[:sql]
  gateway :default

  schema(:news, infer: true) do
    associations do
      has_many        :comments
    end
  end
end

And this is my news repository:

app/repositories/news_repository.rb

class NewsRepository < ApplicationRepository
  root :news

  def all
    root.to_a
  end

  def for_id(id)
    root.where(id: id)
  end

  def with_comments
    root.combine(:comments)
  end

  def one_with_comments!(id)
    root.by_pk(id).combine(:comments).one!
  end
end

I want to get news with comments like that:

[{ 
  id: 1,
  title: 'News title', 
  body: 'News body', 
  comments: [{ 
    id: 1, 
    body: 'Comment 1 body', 
    news_id: 1, 
    user_id: 1, 
    parent_comments_id: nil, 
    comments: [{ 
        id: 2, 
        body: 'Comment 1 child', 
        news_id: 1, 
        user_id: 1, 
        parent_comments_id: 1, 
        comments: [{ 
            id: 3, 
            body: 'Comment 1 child's comment', 
            news_id: 1, 
            user_id: 1, 
            parent_comments_id: 2 
        }] 
    }] 
}]

I attached the diagram:

Screenshot
Screenshot751×608 20.9 KB

The question is also where do I need to get this data?
In CommentsRepository or in the NewsRepository ?

I suspect that I will have to write a custom SQL request. But perhaps it can be done differently.
I will be glad for any answer or help.
Thanks in advance)