Set the where clause for the relation query.

        :return: self
        :rtype: BelongsToMany

def _set_where(self):
        """
        Set the where clause for the relation query.

        :return: self
        :rtype: BelongsToMany
        """
        foreign = self.get_foreign_key()

        self._query.where(foreign, "=", self._parent.get_key())

        return self