Rabl-Rails has_many relation render array of IDs

object.json.rabl

# object.json.rabl
# This is not regular rabl template
# This example is using rabl-rails gem: https://github.com/ccocchi/rabl-rails

object :@product
attributes :id, :name, :description

node :categories do |item|
  item.categories.map do |category|
    category.id
  end
end

output.json

/* api render */

{
  product: {
    id: 1,
    name: "name of product",
    description: "Desc of product",
    categories: [1,2,3]
  }
}