CAN A MOABITE ENTER THE CONGREGATION OF THE LORD?

Deuteronomy 23:3  An Ammonite or Moabite shall not enter into the congregation of the LORD; even to their tenth generation shall they not enter into the congregation of the LORD for ever: (KJV) However, David was the great-grandson of Ruth, a Moabite (See Ruth 1).

First of all, the Deuteronomy passage is a rule, not a prophecy.  So even if David unlawfully entered the congregation, that would not constitute a Biblical contradiction.  It would only constitute an example of a Biblical character doing something he shouldn't have done (and God knows David did many things he shouldn't have done!).  But besides that, nationality was (to the Israelites) usually determined along the paternal line.  David would only be considered part Moabite if his father, paternal grandfather, etc. was a Moabite.  Not a contradiction.