add test

test/atomics_test.exs

@@ -1,4 +1,5 @@
 defmodule AshPostgres.AtomicsTest do
+  alias AshPostgres.Test.Comment
   alias AshPostgres.Test.Author
   use AshPostgres.RepoCase, async: false
   alias AshPostgres.Test.Post
@@ -291,4 +292,24 @@ defmodule AshPostgres.AtomicsTest do
              |> Ash.bulk_update!(:set_title_from_author, %{}, return_records?: true)
              |> Map.get(:records)
   end
+
+  test "can use list aggregate in validation" do
+    post =
+      Post
+      |> Ash.Changeset.for_create(:create, %{title: "foo", price: 1})
+      |> Ash.create!()
+
+    Comment
+    |> Ash.Changeset.for_create(:create, %{post_id: post.id, title: "foo"})
+    |> Ash.create!()
+    |> dbg()
+
+    Logger.configure(level: :debug)
+
+    assert_raise Ash.Error.Invalid, ~r/Can only delete if Post has no comments/, fn ->
+      post
+      |> Ash.Changeset.for_destroy(:destroy_if_no_comments, %{})
+      |> Ash.destroy()
+    end
+  end
 end

test/support/resources/post.ex

@@ -13,6 +13,17 @@ defmodule PassIfOriginalDataPresent do
   end
 end
 
+defmodule HasNoComments do
+  alias Ash.Error.Invalid
+  use Ash.Resource.Validation
+
+  def atomic(_changeset, _opts, _context) do
+    # This uses the list aggregate because we want to specifically test this aggregate
+    {:atomic, [], expr(list(comments, field: :id) > 0),
+     expr(error(^Invalid, %{message: "Can only delete if Post has no comments"}))}
+  end
+end
+
 defmodule AshPostgres.Test.Post do
   @moduledoc false
   use Ash.Resource,
@@ -92,6 +103,10 @@ defmodule AshPostgres.Test.Post do
       change(filter(expr(title == "fred")))
     end
 
+    destroy :destroy_if_no_comments do
+      validate(HasNoComments)
+    end
+
     update :update_only_freds do
       change(filter(expr(title == "fred")))
     end