AE
Ash Elixir•2y ago
Stefan Wintermeyer

How to autoload a relationship?

The setup: lib/app/shop/resources/product.ex 
defmodule App.Shop.Product do
  [...]
  relationships do
    belongs_to :category, App.Shop.Category do
      attribute_writable? true
    end
  end
  [...]
end
defmodule App.Shop.Product do
  [...]
  relationships do
    belongs_to :category, App.Shop.Category do
      attribute_writable? true
    end
  end
  [...]
end
I want to query for the product "Banana": 
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",
   ...
 >}
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",
   ...
 >}
Question: How can I autoload the category when I query for a product so that I don't have to do two SQL requests? App.Shop.Product.by_name("Banana") should get me the product and the category.
3 Replies
barnabasj
barnabasj•2y ago
You can create a read function with a preparation that always adds the load 
actions do
  read :read do
    prepare build(load: [:category])
  end
end
actions do
  read :read do
    prepare build(load: [:category])
  end
end
Or you can do it when you call the interface: 
App.Shop.Product.by_name("Banana", load: [:category])
App.Shop.Product.by_name("Banana", load: [:category])
ZachDaniel
ZachDaniel•2y ago
You can’t do it with only one sql query at the moment though At some point we will add that optimization to Ash to use joins for single cardinality relationships, but it won’t be something you can turn on or off.
kernel
kernel•2y ago
or you can do it for all reads by using a preparations block in your resource 
preparations do
  prepare build(load: [:category])
end
preparations do
  prepare build(load: [:category])
end
many ways to skin cats in Ash 🙂

Did you find this page helpful?