Deuteronomy 21-23

In Deuteronomy 23:3-4 the descendants of Moab were excluded from Israel for 10 generations. Some scholar see this phrasing as a reference to a perpetual prohibition. Yet when we look forward King David we see that only three generations separated him from Ruth the Moabite. How do we explain that exemption? Was it due to David's faith? Was it because Ruth was a woman? Was it her previous marriage to the son of Naomi? Was it because Ruth forsook her family heritage in favor of the God of Israel? Or maybe it is another demonstration of God's grace and providence superseding his nature's demand for justice. I don't this we should neglect God's grace from the picture.
Stuart