Set the base constraints on the relation query.

        :rtype: None

def add_constraints(self):
        """
        Set the base constraints on the relation query.

        :rtype: None
        """
        if self._constraints:
            foreign_key = getattr(self._parent, self._foreign_key, None)
            if foreign_key is None:
                self._query = None
            else:
                table = self._related.get_table()

                self._query.where(
                    "{}.{}".format(table, self._other_key), "=", foreign_key
                )