Deuteronmy 23-25

In Deuteronmy 23:3 God says that for ten generation no one descended from a Moabite may come into God's assembly. How does that apply to David? His great-grandmother was Ruth, a Moabite. One note I read said that the 10 generations could be a euphemism for "never" though not necessarily eternity.  For some reason God allowed David and his descendants not only into his presence but to build his temple.
Stuart