test: add test reproducing related aggregate reference

test/atomics_test.exs

@@ -95,6 +95,23 @@ defmodule AshPostgres.AtomicsTest do
     |> Ash.update!()
   end
 
+  test "an atomic update can be set to the value of a related aggregate" do
+    author =
+      Author
+      |> Ash.Changeset.for_create(:create, %{first_name: "John", last_name: "Doe"})
+      |> Ash.create!()
+
+    post =
+      Post
+      |> Ash.Changeset.for_create(:create, %{title: "bar", author_id: author.id})
+      |> Ash.create!()
+
+    # just asserting that there is no exception here
+    post
+    |> Ash.Changeset.for_update(:set_title_to_author_profile_description)
+    |> Ash.update!()
+  end
+
   test "an atomic validation is based on where it appears in the action" do
     post =
       Post

test/support/resources/post.ex

@@ -100,6 +100,10 @@ defmodule AshPostgres.Test.Post do
       change(atomic_update(:title, expr("#{sum_of_author_count_of_posts}")))
     end
 
+    update :set_title_to_author_profile_description do
+      change(atomic_update(:title, expr(author.profile_description)))
+    end
+
     destroy :destroy_with_confirm do
       require_atomic?(false)
       argument(:confirm, :string, allow_nil?: false)