sqlalchemy foreign key查询和backref

首先在mysql中创建两个表如下：

mysql> create table user( id int,name varchar(8) , primary key(id));
Query OK, 0 rows affected (0.01 sec)

mysql> create table addr(id int,val varchar(100),user_id int, primary key(id),foreign key(user_id)  references user(id) );
Query OK, 0 rows affected (0.00 sec)

mysql> insert into user values(8,‘kramer‘);
Query OK, 1 row affected (0.00 sec)

mysql> insert into user values (18,‘Tom‘);
Query OK, 1 row affected (0.00 sec)

mysql> insert into addr values(1,‘peking‘,8);
Query OK, 1 row affected (0.00 sec)

然后我们用 sqlacodegen 来生成对应的 class。

[email protected]:/opt# sqlacodegen --schema rdb  mysql://root:[email protected]:3306
# coding: utf-8
from sqlalchemy import Column, ForeignKey, Integer, String, Table, text
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()
metadata = Base.metadata

class Addr(Base):
    __tablename__ = ‘addr‘
    __table_args__ = {u‘schema‘: ‘rdb‘}

    id = Column(Integer, primary_key=True, server_default=text("‘0‘"))
    val = Column(String(100))
    user_id = Column(ForeignKey(u‘rdb.user.id‘), index=True)

    user = relationship(u‘User‘)

t_mgr = Table(
    ‘mgr‘, metadata,
    Column(‘id‘, Integer, nullable=False),
    Column(‘name‘, String(18)),
    schema=‘rdb‘
)

class User(Base):
    __tablename__ = ‘user‘
    __table_args__ = {u‘schema‘: ‘rdb‘}

    id = Column(Integer, primary_key=True, server_default=text("‘0‘"))
    name = Column(String(8))

要注意的是原来该数据库中还有个表mgr，但是没有生成class而是生成一个table。这是因为它没有primary key。

接下来我们把生成的代码保存成models.py文件然后操作。

from models import *from sqlalchemy import *db=create_engine(‘mysql://root:[email protected]:3306/rdb?charset=utf8‘,encoding = "utf-8",echo =True)

from sqlalchemy.orm import sessionmaker

S=sessionmaker(bind=db)

s=S()

u=s.query(User).first()
u.addr

AttributeError: ‘User‘ object has no attribute ‘addr‘

u.Addr

AttributeError: ‘User‘ object has no attribute ‘Addr‘a=s.query(Addr).first()
a.user
Out[11]: <models.User at 0xa12e88c>

可以看见通过user来获取addr获取不到，但是通过addr获取user可以。这是因为 addr 下面的代码

user = relationship(u‘User‘)

这段代码说明addr可以通过这个函数来找到对应的user

我们改一下models.py 。把这行代码改成user = relationship(u‘User‘,backref=backref(‘addr‘))就可以通过user来找addr了。新的代码说明，user可以通过backref找到addr

要注意得import sqlalchemy.orm.backref

[email protected]:/opt/temp# cat b.py
# coding: utf-8
from sqlalchemy import Column, ForeignKey, Integer, String, Table, text
from sqlalchemy.orm import relationship,backref
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()
metadata = Base.metadata

class Addr(Base):
    __tablename__ = ‘addr‘
    __table_args__ = {u‘schema‘: ‘rdb‘}

    id = Column(Integer, primary_key=True, server_default=text("‘0‘"))
    val = Column(String(100))
    user_id = Column(ForeignKey(u‘rdb.user.id‘), index=True)

    #user = relationship(u‘User‘)
    user = relationship(u‘User‘,backref=backref(‘addr‘))

t_mgr = Table(
    ‘mgr‘, metadata,
    Column(‘id‘, Integer, nullable=False),
    Column(‘name‘, String(18)),
    schema=‘rdb‘
)

class User(Base):
    __tablename__ = ‘user‘
    __table_args__ = {u‘schema‘: ‘rdb‘}

    id = Column(Integer, primary_key=True, server_default=text("‘0‘"))
    name = Column(String(8))

红色部分是改过的代码，注意有两处

下面用python调用

In [1]: from b import *

In [2]: from sqlalchemy import *

In [3]: db=create_engine(‘mysql://root:[email protected]:3306/rdb?charset=utf8‘,encoding = "utf-8",echo =True)

In [4]: from sqlalchemy.orm import sessionmaker

In [5]: S=sessionmaker(bind=db)

In [6]: s=S()

In [7]: u=s.query(User).first()

In [8]: u.addr

　Out[8]: [<b.Addr at 0xab31c6c>]

In [10]: a.user
Out[10]: <b.User at 0xab3186c>