I have Product and ProductType

class ProductType < ApplicationRecord
    has_many :products
end

class Product < ApplicationRecord
    belongs_to :product_type

    def type_name
        self.product_type.name
    end
end

I can use “type_name” in Product index view to show the product type for each product

but I cannot use type_name to have a Product collection ordered by type_name like this

I tried to search both S.O. and google in general but it seems very few have this quite common problem, and I tried the proposed solutions (using scope or Arel) but with no success