fix: recompute changed? after setup_managed_belongs_to_relationships (#2647)

When manage_relationship is called from before_transaction or
before_action hooks on update actions, the FK attribute gets set via
force_change_attribute inside setup_managed_belongs_to_relationships.
However, the changed? flag was computed before this point, so it
remained false, causing the DB update to be skipped entirely. The
related record was created but the source record's FK was never
persisted.

Recompute changed? after setup_managed_belongs_to_relationships returns
so that FK changes from managed relationships are detected.

Author: barnabasJ

lib/ash/actions/update/update.ex

@@ -471,8 +471,22 @@ defmodule Ash.Actions.Update do
                 {:error, error}
 
               {changeset, manage_instructions} ->
+                # Recompute changed? after setup_managed_belongs_to_relationships
+                # may have set FK attributes via force_change_attribute (e.g. when
+                # manage_relationship is called from before_transaction hooks on
+                # update actions). Without this, the changed? flag computed before
+                # the func callback would be false, causing the DB update to be
+                # skipped entirely.
+                changed? =
+                  Ash.Changeset.changing_attributes?(changeset) or
+                    not Enum.empty?(changeset.atomics)
+
                 changeset =
                   changeset
+                  |> Ash.Changeset.put_context(
+                    :changed?,
+                    changeset.context[:changed?] || changed?
+                  )
                   |> Ash.Changeset.require_values(
                     :update,
                     true

test/manage_relationship_test.exs

@@ -252,6 +252,22 @@ defmodule Ash.Test.ManageRelationshipTest do
 
         change manage_relationship(:parent_resource, on_match: :update)
       end
+
+      update :update_create_parent_in_hook do
+        require_atomic? false
+
+        change fn changeset, _context ->
+          Ash.Changeset.before_transaction(changeset, fn changeset ->
+            Ash.Changeset.manage_relationship(
+              changeset,
+              :parent_resource,
+              %{name: "created-in-hook"},
+              type: :create,
+              on_no_match: :create
+            )
+          end)
+        end
+      end
     end
 
     attributes do
@@ -1289,4 +1305,25 @@ defmodule Ash.Test.ManageRelationshipTest do
       assert names == ["new1", "existing1", "new2", "existing2"]
     end
   end
+
+  describe "manage_relationship called from before_transaction hook on update" do
+    test "creates related record via belongs_to when manage_relationship is called in before_transaction" do
+      related =
+        RelatedResource
+        |> Ash.Changeset.for_create(:create, %{required_attribute: "test"})
+        |> Ash.create!()
+
+      assert related.parent_resource_id == nil
+
+      updated =
+        related
+        |> Ash.Changeset.for_update(:update_create_parent_in_hook, %{})
+        |> Ash.update!()
+
+      assert updated.parent_resource_id != nil
+
+      {:ok, loaded} = Ash.load(updated, :parent_resource)
+      assert loaded.parent_resource.name == "created-in-hook"
+    end
+  end
 end