Return a struct of a given type from a calculation?

This was almost right, This is correct:

def load(records, _, %{api: api, actor: %{id: actor_id}}) do
    {:ok, records
    |> MyApi.load!([:sender, :recipient])
    |> Map.new(fn record -> 
      {record.id, [record.sender, record.recipient] |> Enum.filter(&(&1.id == actor_id))}
    end)}
  end

I was most worried about this making one query per result but it actually doesn’t and works very nicely!

[debug] QUERY OK source="users" db=0.3ms idle=1855.7ms
SELECT u0."id", u0."email", u0."hashed_password" FROM "users" AS u0 LIMIT $1 [1]
↳ AshPostgres.DataLayer.run_query/2, at: lib/data_layer.ex:648
[debug] QUERY OK source="conversations" db=0.5ms idle=1856.8ms
SELECT c0."id", c0."sender_id", c0."recipient_id" FROM "conversations" AS c0 WHERE ((c0."sender_id"::uuid = $1::uuid) OR (c0."recipient_id"::uuid = $2::uuid)) ["d065715d-14fb-4eeb-9e4c-867b41e7ea18", "d065715d-14fb-4eeb-9e4c-867b41e7ea18"]
↳ AshPostgres.DataLayer.run_query/2, at: lib/data_layer.ex:648
[debug] QUERY OK source="users" db=1.2ms idle=1859.4ms
SELECT u0."id", u0."email", u0."hashed_password" FROM "users" AS u0 WHERE (u0."id"::uuid IN ($1::uuid,$2::uuid,$3::uuid,$4::uuid)) ["80cef98a-ed05-4955-9fce-114b54467d4c", "834e6d70-7a48-4065-ae1c-bc8f6c8fbc0e", "99c67645-2208-4185-9961-7517d4779222", "d065715d-14fb-4eeb-9e4c-867b41e7ea18"]
↳ AshPostgres.DataLayer.run_query/2, at: lib/data_layer.ex:648
[debug] QUERY OK source="users" db=0.4ms queue=0.3ms idle=1860.1ms
SELECT u0."id", u0."email", u0."hashed_password" FROM "users" AS u0 WHERE (u0."id"::uuid IN ($1::uuid,$2::uuid,$3::uuid,$4::uuid,$5::uuid,$6::uuid,$7::uuid,$8::uuid,$9::uuid,$10::uuid,$11::uuid,$12::uuid,$13::uuid,$14::uuid,$15::uuid,$16::uuid,$17::uuid,$18::uuid,$19::uuid,$20::uuid,$21::uuid,$22::uuid,$23::uuid,$24::uuid,$25::uuid,$26::uuid,$27::uuid,$28::uuid,$29::uuid,$30::uuid,$31::uuid,$32::uuid,$33::uuid,$34::uuid,$35::uuid,$36::uuid,$37::uuid,$38::uuid,$39::uuid,$40::uuid,$41::uuid,$42::uuid,$43::uuid,$44::uuid,$45::uuid,$46::uuid,$47::uuid,$48::uuid,$49::uuid,$50::uuid,$51::uuid,$52::uuid,$53::uuid,$54::uuid,$55::uuid,$56::uuid,$57::uuid,$58::uuid,$59::uuid,$60::uuid,$61::uuid,$62::uuid,$63::uuid)) ["4e42a0a2-f982-422a-bef3-f77bc3fab5ed", "fbee0b56-50a6-45fb-b9e0-b48103f82f61", "2c97d01a-c00b-4186-9ac2-70f649cad75b", "5150363b-d8cd-4f41-9c81-06b3bcc5faae", "6cfbfa16-4283-478b-bb7e-9d39ddc02c4e", "ce9afdd2-6575-4422-ac4d-b3ffe34aab5c", "941a544a-0c7c-4a72-a531-261706bd6b41", "f8b927a5-27b4-4258-8fef-87b06aec40af", "80cef98a-ed05-4955-9fce-114b54467d4c", "2febf7fc-2e7e-4d10-8e8e-42980bfee682", "4cee3e89-2c4b-4641-9a68-43b4cfc3f629", "9370c439-9ef0-40a3-a545-d10ff68bfa14", "9ed08237-e33f-4282-a9b4-b8c581a0c46e", "bd8b24c9-2202-44fc-9e1e-4b3e92acdbb4", "1412e573-5106-4bcb-aa05-6edbfb9c1040", "28fac61f-bdd7-4a52-bbb1-fc8596056648", "f57bdfe1-aeee-41c1-8ac6-e0af2a0bb848", "49eee110-1c95-4c59-bbda-6904b66b5acb", "cbbae368-03a4-4b3b-a5dc-be6f0eebd0f8", "dc80d246-57f1-43d0-abd5-5138678acfa7", "81bb4fe1-90b8-4a88-9473-0bff35a127c9", "d2b8ffd8-0c95-4aad-9786-a5ac10f07c40", "5f3597f4-740f-41c2-a307-b0353f195c7d", "a9169382-ad06-4477-8b6c-ac4b60c43003", "2d4840dd-2d18-4158-838a-47e3060a9e13", "04ebfc1c-56b0-48a7-92b6-faa02a7cefeb", "25efce68-88a7-4f41-a161-73a0e6f9d4ef", "544fec19-ef81-4dab-8c27-187d3df0a04a", "834e6d70-7a48-4065-ae1c-bc8f6c8fbc0e", "b9567c52-c635-410d-abc0-2f8a3b7d07a2", "50272c49-0640-491e-8645-2dcdacda5f53", "db816428-8204-4270-b077-fa7213e85514", "82d054fc-5a82-4abc-9f21-e5c62d8461da", "99c67645-2208-4185-9961-7517d4779222", "b073f1a9-bc07-4f83-81d4-520b036ad651", "b3d32c08-82c9-4fc8-a7f2-d46d89bd211a", "064d3055-45b2-4f92-b3f5-0ddeb24382a6", "53415340-08d7-4945-bd54-344a45f29a41", "3f258315-194a-4b2e-a031-9d5638e0fe4c", "b4ee2c5a-06b3-45b0-b0f1-31c94a1dae4c", "de893910-4994-438a-a02b-2db7bee5cdca", "ca120785-7c9b-43a4-974a-a98e5d0a9084", "77de8899-43c3-43d2-a2ff-dcb97439c7f8", "85309f25-a09b-4182-8152-ae6cde0a41ba", "c39f04d5-6107-4f3e-a637-2661d62910e7", "d065715d-14fb-4eeb-9e4c-867b41e7ea18", "103c790d-5d2d-4888-8a41-800cd97bf6ed", "e97c3bdb-5f6b-44a0-a1f9-a4be3f0b29cf", "7d6f7f58-ab20-4309-a7f0-9d39f76c6663", "913e38d9-562e-4a49-8089-3b5d0602be28", ...]

So, you won’t be able to this using an expression calculation, you’ll need to use an Elixir calculation. This is because expression calculations done in the data layer can’t return related resources, only relationships and runtime calculations can do that.

defmodule RecipientOrSender do
  use Ash.Calculation

  def load(_, _, _), do: [:recipient, :sender]
  
  def calculate(records, _, context) do
     Enum.map(records, fn record -> 
       if record.recipient.id == context.current_user_id do
         record.recipient
       else
         record.sender
       end
     end)
  end
end

Then you can do:

    calculate :other_user,
              :struct,
              expr(
                if sender_id == ^arg(:current_user_id) do
                  recipient
                else
                  sender
                end
              ) do
      constraints [instance_of: MyApp.Accounts.User]
      argument :current_user_id, :uuid, allow_nil?: false
    end

Although, come to think of it, you could use a manual relationship, assuming you’re okay with using the actor in the standard way as opposed to passing in a current_user_id.

defmodule OtherUser do
  use Ash.Resource.ManualRelationship

  def load(records, _, %{api: api, actor: %{id: actor_id}}) do
    records
    |> Api.load!([:sender, :recipient])
    |> Map.new(fn record -> 
      {record.id, [record.sender, record.recipient] |> Enum.filter(&(&1.id == actor_id))}
    end)
  end

  def load(_, _, _), do: []
end

Haven’t actually run the above code snippets, they are just examples to get you going.