Flask框架之SQLALchemy实现外键及其四种约束

表关系

表之间的关系存在三种：一对一、一对多、多对多。
而SQLAlchemy中的ORM也可以模拟这三种关系。
一对一其实在SQLAlchemy中底层是通过一对多的方式模拟的。

---

SQLAlchemy创建外键

在从表中增加一个字段，指定这个字段外键的是哪个表的哪个字段就可以了。
从表中外键的字段，必须和主表的主键字段类型保持一致。

# 主表(user) / 从表(news) 

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

	def __repr__(self): 
		return "<User(uname:%s)>" % self.uname 

class News(Base): 
	__tablename__ = 'news' 
	id = Column(Integer,primary_key=True,autoincrement=True) 
	title = Column(String(50),nullable=False) 
	content = Column(Text,nullable=False) 
	uid = Column(Integer,ForeignKey("user.id")) 

	def __repr__(self): 
	return "<News(title:%s,content=%s)>" % (self.title,self.content)
	

---

外键约束

1. RESTRICT：若子表中有父表对应的关联数据，删除父表对应数据，会阻止删除。默认项
2. NO ACTION：在MySQL中，同RESTRICT。
3. `CASCADE：级联删除。
4. SET NULL：父表对应数据被删除，子表对应数据项会设置为NULL。

from sqlalchemy import create_engine,Column,Integer,Float,Boolean,DECIMAL,Enum,\
	 Date,DateTime,Time,String,Text,func,or_,and_,ForeignKey  
from sqlalchemy.dialects.mysql import LONGTEXT 
from sqlalchemy.ext.declarative import declarative_base 
from sqlalchemy.orm import sessionmaker 
import random 

HOSTNAME = '127.0.0.1' 
PORT = '3306' 
DATABASE = 'first_sqlalchemy' 
USERNAME = 'root' 
PASSWORD = 'root' 
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)() 

# 父表/从表 # user/news 
class User(Base): 
	__tablename__ = 'user' 
	id = Column(Integer,primary_key=True,autoincrement=True) 
	uname = Column(String(50),nullable=False) 

	def __repr__(self): 
		return "<User(uname:%s)>" % self.uname 

class News(Base): 
	__tablename__ = 'news' 
	id = Column(Integer,primary_key=True,autoincrement=True) 
	title = Column(String(50),nullable=False) 
	content = Column(Text,nullable=False) 
	# uid = Column(Integer,ForeignKey("user.id",ondelete='RESTRICT')) 
	# uid = Column(Integer,ForeignKey("user.id",ondelete='NO ACTION')) 
	# uid = Column(Integer,ForeignKey("user.id",ondelete='CASCADE')) 
	uid = Column(Integer,ForeignKey("user.id",ondelete='SET NULL')) 

	def __repr__(self): 
		return "<News(title:%s,content=%s)>" % (self.title,self.content)

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

user = User(uname='wukong') 

session.add(user) 
session.commit() 

news1= News(title='AAA',content='123',uid=1) 
news2= News(title='BBB',content='456',uid=1) 

session.add_all([news1,news2]) 
session.commit()