field via aliased relationship calculation

lib/diffo/provider/components/calculations/field_via_aliased_relationship.ex

@@ -0,0 +1,42 @@
+# SPDX-FileCopyrightText: 2025 diffo contributors <https://github.com/diffo-dev/diffo/graphs.contributors>
+#
+# SPDX-License-Identifier: MIT
+
+defmodule Diffo.Provider.Calculations.FieldViaAliasedRelationship do
+  @moduledoc false
+  use Ash.Resource.Calculation
+
+  @impl true
+  def load(_query, _opts, _context), do: []
+
+  @impl true
+  def calculate(records, opts, _context) do
+    alias_name = opts[:alias]
+    field = opts[:field]
+
+    Enum.map(records, fn record ->
+      record.id
+      |> traverse(alias_name)
+      |> Enum.flat_map(fn target_id ->
+        Diffo.Provider.Instance
+        |> Ash.Query.filter_input(id: target_id)
+        |> Ash.read!(domain: Diffo.Provider)
+        |> Enum.map(&Map.get(&1, field))
+      end)
+    end)
+  end
+
+  defp traverse(id, nil) do
+    Diffo.Provider.DefinedSimpleRelationship
+    |> Ash.Query.filter_input(source_id: id)
+    |> Ash.read!(domain: Diffo.Provider)
+    |> Enum.map(& &1.target_id)
+  end
+
+  defp traverse(id, alias_name) do
+    Diffo.Provider.DefinedSimpleRelationship
+    |> Ash.Query.filter_input(source_id: id, alias: alias_name)
+    |> Ash.read!(domain: Diffo.Provider)
+    |> Enum.map(& &1.target_id)
+  end
+end

test/provider/extension/field_via_aliased_relationship_test.exs

@@ -0,0 +1,101 @@
+# SPDX-FileCopyrightText: 2025 diffo contributors <https://github.com/diffo-dev/diffo/graphs.contributors>
+#
+# SPDX-License-Identifier: MIT
+
+defmodule Diffo.Provider.Extension.FieldViaAliasedRelationshipTest do
+  @moduledoc false
+  use ExUnit.Case, async: true
+  @moduletag :domain_extended
+
+  alias Diffo.Test.Parties
+  alias Diffo.Test.Servo
+
+  setup do
+    AshNeo4j.Sandbox.checkout()
+    on_exit(&AshNeo4j.Sandbox.rollback/0)
+  end
+
+  describe "FieldViaAliasedRelationship — aliased" do
+    test "returns field from target instance reached via alias" do
+      {:ok, shelf} = Parties.build_shelf_with_installer()
+      {:ok, card} = Servo.build_card(%{name: "target-card"})
+
+      Diffo.Provider.create_defined_simple_relationship!(%{
+        type: :assignedTo,
+        alias: :link,
+        source_id: shelf.id,
+        target_id: card.id
+      })
+
+      shelf = Ash.load!(shelf, [:linked_target_name], domain: Servo)
+
+      assert shelf.linked_target_name == ["target-card"]
+    end
+
+    test "returns empty list when no matching relationship exists" do
+      {:ok, shelf} = Parties.build_shelf_with_installer()
+
+      shelf = Ash.load!(shelf, [:linked_target_name], domain: Servo)
+
+      assert shelf.linked_target_name == []
+    end
+
+    test "alias filters to only the matching target" do
+      {:ok, shelf} = Parties.build_shelf_with_installer()
+      {:ok, card_a} = Servo.build_card(%{name: "target-a"})
+      {:ok, card_b} = Servo.build_card(%{name: "target-b"})
+
+      Diffo.Provider.create_defined_simple_relationship!(%{
+        type: :assignedTo,
+        alias: :link,
+        source_id: shelf.id,
+        target_id: card_a.id
+      })
+
+      Diffo.Provider.create_defined_simple_relationship!(%{
+        type: :assignedTo,
+        alias: :other,
+        source_id: shelf.id,
+        target_id: card_b.id
+      })
+
+      shelf = Ash.load!(shelf, [:linked_target_name], domain: Servo)
+
+      assert shelf.linked_target_name == ["target-a"]
+    end
+  end
+
+  describe "FieldViaAliasedRelationship — unaliased (all targets)" do
+    test "returns fields from all related target instances regardless of alias" do
+      {:ok, shelf} = Parties.build_shelf_with_installer()
+      {:ok, card_a} = Servo.build_card(%{name: "target-a"})
+      {:ok, card_b} = Servo.build_card(%{name: "target-b"})
+
+      Diffo.Provider.create_defined_simple_relationship!(%{
+        type: :assignedTo,
+        alias: :link,
+        source_id: shelf.id,
+        target_id: card_a.id
+      })
+
+      Diffo.Provider.create_defined_simple_relationship!(%{
+        type: :assignedTo,
+        alias: :other,
+        source_id: shelf.id,
+        target_id: card_b.id
+      })
+
+      shelf = Ash.load!(shelf, [:all_linked_names], domain: Servo)
+
+      assert Enum.sort(shelf.all_linked_names) == ["target-a", "target-b"]
+    end
+
+    test "returns empty list when no relationships exist" do
+      {:ok, shelf} = Parties.build_shelf_with_installer()
+
+      shelf = Ash.load!(shelf, [:all_linked_names], domain: Servo)
+
+      assert shelf.all_linked_names == []
+    end
+  end
+end

test/support/resource/instance/shelf_instance.ex

@@ -84,6 +84,14 @@ defmodule Diffo.Test.Instance.ShelfInstance do
     end
   end
 
+  calculations do
+    calculate :linked_target_name, {:array, :string},
+      {Diffo.Provider.Calculations.FieldViaAliasedRelationship, [alias: :link, field: :name]}
+
+    calculate :all_linked_names, {:array, :string},
+      {Diffo.Provider.Calculations.FieldViaAliasedRelationship, [field: :name]}
+  end
+
   actions do
     create :build do
       description "creates a new Shelf resource instance for build"