python---ORM之SQLAlchemy（3）外键与relationship的关系

relationship是为了简化联合查询join等，创建的两个表之间的虚拟关系，这种关系与标的结构时无关的。他与外键十分相似，确实，他必须在外键的基础上才允许使用

不然会报错：

sqlalchemy.exc.NoForeignKeysError: Could not determine join condition between parent/child tables on relationship Father.son - there are no foreign keys linking these tables.  Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression

详细的relationship可以点击这里进行查看

relationship的使用：

使两个表之间产生管理，类似于合成一张表，可以直接取出关联的表，进行获取数据，而不需要join操作

import sqlalchemy
from sqlalchemy import create_engine
from sqlalchemy import Column,String,Integer,ForeignKey
from sqlalchemy.orm import sessionmaker,relationship
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine("mysql+pymysql://root:root@127.0.0.1/t1")

Base = declarative_base()

class Father(Base):
    __tablename__ = "father"

    id = Column(Integer,primary_key=True,autoincrement=True)
    name = Column(String(40),unique=True)
    age = Column(Integer)
    son = relationship('Son',backref="father")

class Son(Base):
    __tablename__ = 'son'

    id = Column(Integer,primary_key=True,autoincrement=True)
    name = Column(String(40),unique=True)
    age = Column(Integer)

    father_id = Column(Integer,ForeignKey('father.id'))

Base.metadata.create_all(engine)

MySession = sessionmaker(bind=engine)
session = MySession()

# f = Father(name='ld',age=21)
# session.add(f)
# session.commit()
#
# s1 = Son(name='ww',age=1,father_id=1)
# s2 = Son(name='wb',age=0,father_id=1)
# session.add_all([s1,s2])
# session.commit()
#一对多情况下：多（包含外键方）

ret =session.query(Father).filter_by(id=1).first()
#ret.son 是一个列表，其中多的一方会获得一个列表结果，列表中含有其各个对象
for i in ret.son:
    print(i.name,i.age)


#另一方只会获得一个对象结果
ret2 = session.query(Son).filter_by(id=1).first()
print(ret2.father.name)#

只使用外键，需要使用join才可以取出数据

#上面不存在relationship
ret = session.query(Father.name.label('kkk'),Son.name.label("ppp")).join(Son).all()#使用Join才可以获取对方数据
print(ret)#是一个列表，列表中存在所要获取的数据（以元组存在）

在外键基础上使用relationship：可以直接通过属性操作获取数据

#使用了relationship
ret = session.query(Father).filter_by(id=1).first()
print(ret.son)#是一个对象列表，其中包含了所有查询数据

 全部代码：

其中son = relationship('Son',backref="Father")

相当于在Son中加入father = relationship('Father')在Father中加入son = relationship('Son')

import sqlalchemy
from sqlalchemy import create_engine
from sqlalchemy import Column,String,Integer,ForeignKey
from sqlalchemy.orm import sessionmaker,relationship
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine("mysql+pymysql://root:root@127.0.0.1/t1")

Base = declarative_base()

class Father(Base):
    __tablename__ = "father"

    id = Column(Integer,primary_key=True,autoincrement=True)
    name = Column(String(40),unique=True)
    age = Column(Integer)
    son = relationship('Son',backref="Father")
    #son = relationship('Son')

class Son(Base):
    __tablename__ = 'son'

    id = Column(Integer,primary_key=True,autoincrement=True)
    name = Column(String(40),unique=True)
    age = Column(Integer)
    #father = relationship('Father')

    father_id = Column(Integer,ForeignKey('father.id'))

Base.metadata.create_all(engine)

MySession = sessionmaker(bind=engine)
session = MySession()

ret = session.query(Father).filter_by(id=1).first()
print(ret.son) #多个结果[<__main__.Son object at 0x0000000003F192B0>, <__main__.Son object at 0x0000000003F19320>]
#需要循环取值

ret = session.query(Son).filter_by(id=1).first()
print(ret.father)#一个结果<__main__.Father object at 0x0000000003F196D8>
#直接取值