test: add test showing no query with exists

test/ash_postgres_test.exs

@@ -1,5 +1,6 @@
 defmodule AshPostgresTest do
   use AshPostgres.RepoCase, async: false
+  import ExUnit.CaptureLog
 
   test "transaction metadata is given to on_transaction_begin" do
     AshPostgres.Test.Post
@@ -40,4 +41,30 @@ defmodule AshPostgresTest do
       |> Map.get(:title)
     end
   end
+
+  test "it does not run queries for exists/2 expressions that can be determined from loaded data" do
+    author = 
+      AshPostgres.Test.Author
+      |> Ash.Changeset.for_create(:create, %{}, authorize?: false)
+      |> Ash.create!()
+
+    post =
+      AshPostgres.Test.Post
+      |> Ash.Changeset.for_create(:create, %{title: "good", author_id: author.id})
+      |> Ash.create!()
+      |> Ash.load!(:author)
+
+    log =
+      capture_log(fn -> 
+        post
+        |> Ash.Changeset.for_update(:update_if_author, %{title: "bad"},
+          authorize?: true,
+          actor: nil,
+          actor: author
+        )
+        |> then(&AshPostgres.Test.Post.can_update_if_author?(author, &1))
+      end)
+
+    assert log == ""
+  end
 end

test/support/resources/post.ex

@@ -80,6 +80,10 @@ defmodule AshPostgres.Test.Post do
       authorize_if(PassIfOriginalDataPresent)
     end
 
+    bypass action(:update_if_author) do
+      authorize_if relates_to_actor_via(:author)
+    end
+
     policy action_type(:update) do
       authorize_if(action(:requires_initial_data))
       authorize_if(relates_to_actor_via([:author, :authors_with_same_first_name]))
@@ -231,6 +235,10 @@ defmodule AshPostgres.Test.Post do
       require_atomic?(false)
     end
 
+    update :update_if_author do
+      require_atomic?(false)
+    end
+
     update(:dont_validate)
 
     update :change_title_to_foo_unless_its_already_foo do
@@ -423,6 +431,7 @@ defmodule AshPostgres.Test.Post do
     define(:get_by_id, action: :read, get_by: [:id])
     define(:increment_score, args: [{:optional, :amount}])
     define(:destroy)
+    define(:update_if_author)
     define(:update_constrained_int, args: [:amount])
 
     define_calculation(:upper_title, args: [:title])