@hybrid_property with method expression

    _quantity = Column('quantity', Numeric, nullable=False)
    quantity_item_id = Column(Integer, ForeignKey('job_item.id'))
    quantity_item = relationship('Job_Item', remote_side=id, backref=backref('children'))

    @hybrid_property
    def quantity(self):
        return self.quantity_item.quantity if self.quantity_item else self._quantity

    @quantity.expression
    def quantity(self, quantity_item):
        return case(
            [(quantity_item.id.isnot(None), quantity_item._quantity)],
            else_=self._quantity
        )*self.carrier_rate