Add reference from back_ref to self

        :param back_ref: back_ref to add
        :type back_ref: Resource

        :rtype: Resource

def add_back_ref(self, back_ref, attr=None):
        """Add reference from back_ref to self

        :param back_ref: back_ref to add
        :type back_ref: Resource

        :rtype: Resource
        """
        back_ref.add_ref(self, attr)
        return self.fetch()