add test to show error with building a reference

test/atomics_test.exs

@@ -198,6 +198,13 @@ defmodule AshPostgres.AtomicsTest do
       |> Ash.update!()
 
     assert post.title == "John"
+
+    post =
+      post
+      |> Ash.Changeset.for_update(:set_attributes_from_parent, %{})
+      |> Ash.update!()
+
+    assert post.title == "John"
   end
 
   test "relationships can be used in atomic update and in an atomic update filter" do

test/support/resources/post.ex

@@ -134,6 +134,23 @@ defmodule AshPostgres.Test.Post do
       end)
     end
 
+    update :set_attributes_from_parent do
+      require_atomic?(false)
+
+      change(
+        atomic_update(
+          :title,
+          expr(
+            if is_nil(parent_post_id) do
+              author.title
+            else
+              parent_post.author.title
+            end
+          )
+        )
+      )
+    end
+
     update :update do
       primary?(true)
       require_atomic?(false)
@@ -273,6 +290,10 @@ defmodule AshPostgres.Test.Post do
       filter(expr(^actor(:id) == id))
     end
 
+    belongs_to :parent_post, __MODULE__ do
+      public?(true)
+    end
+
     belongs_to(:author, AshPostgres.Test.Author) do
       public?(true)
     end