python 之sqlalchemy many to one

通过查询多个父亲，对应一个儿子

 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 """
 @author: zengchunyun
 """
 from sqlalchemy import Column, Integer, String, Float, DateTime, ForeignKey
 from sqlalchemy.orm import sessionmaker, relationship, backref
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy import create_engine

 Base = declarative_base()
 engine = create_engine('mysql+pymysql://root:123@127.0.0.1:3306/day11',echo=True)

 class Parent(Base):
     __tablename__ = 'parent'
     id = Column(Integer, primary_key=True)
     name = Column(String(64))
     child_id = Column(Integer, ForeignKey("child.id"))
     childr = relationship("Child")

 class Child(Base):
     __tablename__ = 'child'
     id = Column(Integer, primary_key=True)
     name = Column(String(64))

 Base.metadata.create_all(engine)

 DBSession = sessionmaker()
 DBSession.configure(bind=engine)
 session = DBSession()  # 打开数据连接

 ret = session.query(Parent).filter(Parent.name == 'zeng').one()
 print(ret)
 print(ret.childr.name)
 ret = session.query(Parent).filter(Parent.name == 'chunyun').one()
 print(ret)
 print(ret.childr.name)

 ret = session.query(Parent).filter(Parent.name == 'chun').one()
 print(ret)
 print(ret.childr.name)

many to one 第二式

 class Parent(Base):
     __tablename__ = 'parent'
     id = Column(Integer, primary_key=True)
     name = Column(String(64))
     child_id = Column(Integer, ForeignKey("child.id"))
     childr = relationship("Child", back_populates="parents")

 class Child(Base):
     __tablename__ = 'child'
     id = Column(Integer, primary_key=True)
     name = Column(String(64))
     parents = relationship("Parent", back_populates="childr")找到父亲

 ＃通过双向建立映射关系，能让父亲查到儿子，也能让儿子

many to one 终极版

 class Parent(Base):
     __tablename__ = 'parent'
     id = Column(Integer, primary_key=True)
     name = Column(String(64))
     child_id = Column(Integer, ForeignKey("child.id"))
     childr = relationship("Child", backref="parents")  # 这段代码,变相的等于在Child类中添加了parents = relationship("Parent", back_populates="childr")

 class Child(Base):
     __tablename__ = 'child'
     id = Column(Integer, primary_key=True)
     name = Column(String(64))