Add handling of objects not in database

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -311,8 +311,14 @@ def delete(%Object{data: %{"id" => id, "actor" => actor}} = object, local \\ tru
     user = User.get_cached_by_ap_id(actor)
 
     to =
+      case Object.get_cached_by_ap_id(id) do
+        nil ->
+          [user.follower_address, "https://www.w3.org/ns/activitystreams#Public"]
+
+        object ->
           object.data["to"] || [] ++ object.data["cc"] ||
             [] ++ [user.follower_address, "https://www.w3.org/ns/activitystreams#Public"]
+      end
 
     data = %{
       "type" => "Delete",

test/web/activity_pub/activity_pub_test.exs

@@ -695,8 +695,19 @@ test "decrements user note count only for public activities" do
     test "it creates a delete activity and checks that it is also sent to users mentioned by the deleted object" do
       user = insert(:user)
       note = insert(:note_activity)
-      object = Object.get_by_ap_id(note.data["object"]["id"])
-      object = Kernel.put_in(object.data["to"], [user.ap_id])
+
+      {:ok, object} =
+        Object.get_by_ap_id(note.data["object"]["id"])
+        |> Object.change(%{
+          data: %{
+            "actor" => note.data["object"]["actor"],
+            "id" => note.data["object"]["id"],
+            "to" => [user.ap_id],
+            "type" => "Note"
+          }
+        })
+        |> Object.update_and_set_cache()
+
       {:ok, delete} = ActivityPub.delete(object)
 
       assert user.ap_id in delete.data["to"]