sqlalchemy里面many----one关联的级联删除设置如何解决？

(many) (one)
Student Klass

class Student(Base):
tablename = 'student'
id = Column(Integer, primary_key=True)
name = Column(String(255))
klass_id = Column(Integer, ForeignKey('klass.id'), nullable=False)
klass = relationship('Klass', backref=backref('students', cascade="all, delete-orphan"))

class Klass(Base):
tablename = 'klass'
id = Column(Integer, primary_key=True)
name = Column(String(255), nullable=False)

要求：
删除student端数据A的时候，如果student是唯一的和klass关联的对象，那么同时删除klass对象。

参考文档： http://docs.sqlalchemy.org/en/rel_0_9/orm/session.html#cascades --- 没有弄明白，小白呀