Boaz is the type of the bridegroom who marries Ruth in order to raise up a seed for the old and widowed Naomi.  James Jordan says Ruth is a substitute bride, because the firstborn seed is her seed, and leads to the redeemer that comes into the world.  Typologically: The Bridegroom marries the Moabitess, the Gentile widow, but he does it in order to give a seed to the Israelite widow.  All Israel is saved through the marriage of Gentiles with the redeemer.