级联： class Witer(db.Model): books=db.relationship('writer',cascade='save-update,merge,delete')
save-update,merge
all all,delete-orphan
尽量不要设置 delete-orphan
事件监听： set append remove init_scalar init_collection
当字段被重新编辑时，记录被编辑的次数
设置
@db.event.listens_for(Post.body,'set') def increment_edit_time(target,value,oldvalue,initiator): if target.edit_time is not None: target.edit_time =1
or 字典方式：
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