python多对多删除中间表

[jijihahalala]

我定义了多对多关系如下：


classroom_student = Table('class_student', Base.metadata,

Column('classroom_id', Integer, ForeignKey('classroom.id')),
Column('student_id', Integer, ForeignKey('student.id')),
)

class ClassRoom(Base):
__tablename__ = 'classroom'
id = Column(Integer, primary_key=True, nullable=False, autoincrement=True)
name = Column(String(20))
def __init__(self, name):
self.name = name


class Student(Base):
__tablename__ = 'student'
id = Column(Integer, primary_key=True, nullable=False, autoincrement=True)
name = Column(String(20))
classId_studentId = relationship("ClassRoom",
secondary=classroom_student,
backref=backref('classroom_student',cascade="delete, delete-orphan", single_parent=True, passive_deletes=True))
def __init__(self, name):
self.name = name


我想删除ClassRoom或者删除Student时删除class_student该怎么设置，删除ClassRoom时不要删除Student，谢谢回答