sqlalchemy orm 层面删除数据注意

#encoding: utf-8

from sqlalchemy import create_engine,Column,Integer,String,Float,func,and_,or_,Text,
    ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker,relationship,backref
from random import randint

HOSTNAME = '127.0.0.1'

PORT = 3306

DATABASE = 'first_sqlalchemy'

USERNAME = 'root'

PASSWORD = '123456'

#dialect+driver://username:password@host:port/database
DB_URI = "mysql+pymysql://{username}:{password}@{host}:{port}/" 
         "{db}?charset=utf8".format(username=USERNAME,password=PASSWORD,host=HOSTNAME,port=PORT,db=DATABASE)

engine = create_engine(DB_URI)

Base = declarative_base(engine)

# Session = sessionmaker(engine)
# session = Session()

session = sessionmaker(engine)() #Session(**local_kw)


class User(Base):
    __tablename__ = 'user'
    id = Column(Integer,primary_key=True,autoincrement=True)
    username = Column(String(50),nullable=False,unique=True)


class Article(Base):
    __tablename__ = 'article'
    id = Column(Integer,primary_key=True,autoincrement=True)
    article_name = Column(String(50),nullable=False)
    uid = Column(Integer,ForeignKey('user.id'))
    author = relationship('User',backref='articles')

# Base.metadata.drop_all()
#
# Base.metadata.create_all()

#增加数据
# user = User(username='xiaowu')
#
# article = Article(article_name = 'xiaoshuowang')
#
# user.articles.append(article)
#
# session.add(user)
# session.commit()


#删除表中的数据,不是说存外键关系，主表的数据是不能删除的吗？
user = session.query(User).first()
session.delete(user)
session.commit()
#一对多关系，外键约束默认是RESTRICT,父表有关联子表，数据删除不掉，为社么这里不行
#因为在orm层面没有设置uid = Column(Integer,ForeignKey('user.id')) nullable 等于False,
#所以，uid设置成null,父表的主键就可以删除了
#要避免这样的结果就要这样设置，uid = Column(Integer,ForeignKey('user.id'),nullable=False,
#就行了