Python与数据库[2] -> 关系对象映射/ORM[3] -> sqlalchemy 的声明层 ORM 访问方式

sqlalchemy的声明层ORM访问方式

---

sqlalchemy中可以利用声明层进行表格类的建立，并利用ORM对象进行数据库的操作及访问，另一种方式为显式的 ORM 访问方式。

主要的建立步骤包括:

1. 设置DSN连接网址;

2. 创建引擎，利用引擎对DSN进行连接;

3. 创建会话类并绑定到引擎，创建引擎实例;

4. 创建表类，由于使用声明层元数据;

5. 利用会话实例来进行数据库操作。

基本连接关系图:

下面是完整代码

 from sqlalchemy import create_engine, MetaData
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.sql.schema import Table, Column
 from sqlalchemy.sql.sqltypes import Integer, VARCHAR
 from sqlalchemy.orm import sessionmaker

 Base = declarative_base()

 # Set DSN
 dsn = 'mysql+mysqldb://root:root@localhost/test_db'

 # Create engine, get connection
 eng = create_engine(dsn)
 cnx = eng.connect()

 # Make a Session class binded with engine
 Session = sessionmaker(bind=eng)
 # Generate a session instance
 session = Session()

 # Build up Table class
 class Test_tb(Base):
     __table__ = Table('test_tb', Base.metadata,
                         Column('id', Integer, primary_key=True),
                         Column('name', VARCHAR(8))
                         )

 # Insert value
 # session.add_all([Test_tb(id=6, name='Momo')])
 session.add(Test_tb(id=6, name='Momo'))
 session.commit()

 # Delete value
 session.query(Test_tb).filter(Test_tb.id==6).delete()
 session.commit()

 session.add_all([Test_tb(id=7, name='Momo'), Test_tb(id=8, name='Kitkat')])
 session.commit()
 # Update value
 fr = session.query(Test_tb).filter_by(id=7).all()
 for f in fr:
     f.id = 9
 session.query(Test_tb).filter(Test_tb.id==8).delete()
 session.query(Test_tb).filter(Test_tb.id==9).delete()
 session.commit()

 re = session.query(Test_tb).all()
 # Show value: Each r is a row in table
 for r in re:
     print(r.__dict__)

 cnx.close()

分段分析

首先导入所需的模块，创建声明层基类

 from sqlalchemy import create_engine, MetaData
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.sql.schema import Table, Column
 from sqlalchemy.sql.sqltypes import Integer, VARCHAR
 from sqlalchemy.orm import sessionmaker

 Base = declarative_base()

 # Set DSN
 dsn = 'mysql+mysqldb://root:root@localhost/test_db'

创建引擎，以及会话实例

 # Create engine, get connection
 eng = create_engine(dsn)
 cnx = eng.connect()

 # Make a Session class binded with engine
 Session = sessionmaker(bind=eng)
 # Generate a session instance
 session = Session()

基于声明层创建表格类

 # Build up Table class
 class Test_tb(Base):
     __table__ = Table('test_tb', Base.metadata,
                         Column('id', Integer, primary_key=True),
                         Column('name', VARCHAR(8))
                         )

最后利用会话实例对表格进行操作

 # Insert value
 # session.add_all([Test_tb(id=6, name='Momo')])
 session.add(Test_tb(id=6, name='Momo'))
 session.commit()

 # Delete value
 session.query(Test_tb).filter(Test_tb.id==6).delete()
 session.commit()

 session.add_all([Test_tb(id=7, name='Momo'), Test_tb(id=8, name='Kitkat')])
 session.commit()
 # Update value
 fr = session.query(Test_tb).filter_by(id=7).all()
 for f in fr:
     f.id = 9
 session.query(Test_tb).filter(Test_tb.id==8).delete()
 session.query(Test_tb).filter(Test_tb.id==9).delete()
 session.commit()

 re = session.query(Test_tb).all()
 # Show value: Each r is a row in table
 for r in re:
     print(r.__dict__)

 cnx.close()

最终运行结果

{'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x000000000489F320>, 'id': 3, 'name': 'eric'}
{'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x000000000489F390>, 'id': 5, 'name': 'mock'}

相关阅读

---

1. 显式的 ORM 访问方式

2. ORM 与 sqlalchemy 模块

3. 建立声明层表对象的两种方式