How to autoload a relationship?

defmodule App.Shop.Product do
  [...]
  relationships do
    belongs_to :category, App.Shop.Category do
      attribute_writable? true
    end
  end
  [...]
end

iex(12)> fruits = App.Shop.Category.create!(%{name: "Fruits"})
#App.Shop.Category<
  __meta__: #Ecto.Schema.Metadata<:loaded>,
  id: "3a3ed806-6df4-4602-aff6-4eed50e814b2",
  name: "Fruits",
  ...
>
iex(13)> banana = App.Shop.Product.create!(%{name: "Banana", price: 0.1, category_id: fruits.id})
#App.Shop.Product<
  promotion: #Ash.NotLoaded<:relationship>,
  category: #Ash.NotLoaded<:relationship>,
  __meta__: #Ecto.Schema.Metadata<:loaded>,
  id: "d655fa01-ece5-4ad9-b927-764a6a48b165",
  name: "Banana",
  price: Decimal.new("0.1"),
  category_id: "3a3ed806-6df4-4602-aff6-4eed50e814b2",
  ...
>
iex(14)> App.Shop.Product.by_name("Banana")
{:ok,
 #App.Shop.Product<
   promotion: #Ash.NotLoaded<:relationship>,
   category: #Ash.NotLoaded<:relationship>,
   __meta__: #Ecto.Schema.Metadata<:loaded>,
   id: "d655fa01-ece5-4ad9-b927-764a6a48b165",
   name: "Banana",
   price: Decimal.new("0.1"),
   category_id: "3a3ed806-6df4-4602-aff6-4eed50e814b2",
   ...
 >}
iex(15)> App.Shop.load(banana, :category)
{:ok,
 #App.Shop.Product<
   promotion: #Ash.NotLoaded<:relationship>,
   category: #App.Shop.Category<
     __meta__: #Ecto.Schema.Metadata<:loaded>,
     id: "3a3ed806-6df4-4602-aff6-4eed50e814b2",
     name: "Fruits",
     ...
   >,
   __meta__: #Ecto.Schema.Metadata<:loaded>,
   id: "d655fa01-ece5-4ad9-b927-764a6a48b165",
   name: "Banana",
   price: Decimal.new("0.1"),
   category_id: "3a3ed806-6df4-4602-aff6-4eed50e814b2",
   ...
 >}