How to access relationship with JSON:API serializer

Question

I am currently developping an comment system using ruby on rails and React. Everything was working well, my User can post comments, and products can receive comments. But now I have a problem. Here is my three models:

The Product

class Product< ApplicationRecord
  has_many :comments
end

The User

class User < ApplicationRecord
  has_many :comments
end

and the Comment

class Comment < ApplicationRecord
  belongs_to :Product
  belongs_to :user
end

Now I would like to display render the Product on a product page, using JSONAPI::Serializer.

So I call the ProductSerializer:

 def show
    render json: ProductSerializer.new(@product, { include: [:comments] })
  end

and here is the ProductSerializer:

class ProductSerializer
  include JSONAPI::Serializer
  attributes :id, :created_at, :description
  has_many :comments
end

(I followed the official documentation on github here : https://github.com/jsonapi-serializer/jsonapi-serializer)

so now the problem is: why the json response does not include the comments? I have also tried :

class ProductSerializer
  include JSONAPI::Serializer
  attributes :id, :created_at, :description
  has_many :comments, serializer: CommentSerializer
end

But it still does not work... And if I include comments within the attributes of the Serializer, like that:

class ProductSerializer
  include JSONAPI::Serializer
  attributes :id, :created_at, :description, :comments
end

I do receive the list of comment, but instead of an User object, I get an User_id and that is not what I want.

I am using rails 7.0.6 and jsonapi-serializer 2.2.0