Python与数据库[2] -> 关系对象映射/ORM[4] -> sqlalchemy 的显式 ORM 访问方式

sqlalchemy 的显式 ORM 访问方式

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对于sqlalchemy，可以利用一种显式的ORM方式进行访问，这种方式无需依赖声明层，而是显式地进行操作。还有一种访问方式为声明层 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. 创建元数据实例，并绑定到引擎;

              4. 创建表类，并与元数据进行绑定;

              5. 进行数据库操作。

基本连接关系图:

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

# No need to create declarative_base
# Base = declarative_base()

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

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

# No need to create session
# Session = sessionmaker(bind=eng)
# session = Session()

# Generate metadata instance
metadata = MetaData()
# Bind engine to metadata
metadata.bind = eng

# Build up Table
test_tb = Table('test_tb', metadata, 
                Column('id', Integer),
                Column('name', VARCHAR(8))
                )
#print(type(test_tb.insert()))
# Insert value, .rowcount will return the row number affected
print(test_tb.insert().execute(id=6, name='Momo').rowcount)
# Delete value, note: '.c' attr should be added
print(test_tb.delete(test_tb.c.id==6).execute().rowcount)

print(test_tb.insert().execute([{'id': 7, 'name': 'Momo'}, {'id': 8, 'name': 'Kitkat'}]).rowcount)
# Update value, change value where id is 7 to 9
print(test_tb.update(test_tb.c.id==7).execute(id=9).rowcount)

print(test_tb.delete(test_tb.c.id==8).execute().rowcount)
print(test_tb.delete(test_tb.c.id==9).execute().rowcount)

cnx.close()

分段分析

首先导入所需的模块，由于不需要声明层，此处进行注释

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

# No need to create declarative_base
# Base = declarative_base()

设置DSN参数，利用该参数进行引擎实例创建并连接，再创建元数据，与引擎进行绑定，最后建立表格类，与元数据进行绑定。

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

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

# No need to create session
# Session = sessionmaker(bind=eng)
# session = Session()

# Generate metadata instance
metadata = MetaData()
# Bind engine to metadata
metadata.bind = eng

# Build up Table
test_tb = Table('test_tb', metadata, 
                Column('id', Integer),
                Column('name', VARCHAR(8))
                )

进行基本操作测试，注意此处删除时的条件语句在表格名和列名中间需要加入c表示列属性，最后利用连接实例关闭连接。

#print(type(test_tb.insert()))
# Insert value, .rowcount will return the row number affected
print(test_tb.insert().execute(id=6, name='Momo').rowcount)
# Delete value, note: '.c' attr should be added
print(test_tb.delete(test_tb.c.id==6).execute().rowcount)

print(test_tb.insert().execute([{'id': 7, 'name': 'Momo'}, {'id': 8, 'name': 'Kitkat'}]).rowcount)
# Update value, change value where id is 7 to 9
print(test_tb.update(test_tb.c.id==7).execute(id=9).rowcount)

print(test_tb.delete(test_tb.c.id==8).execute().rowcount)
print(test_tb.delete(test_tb.c.id==9).execute().rowcount)

cnx.close()

相关阅读

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1. 声明层 ORM 访问方式

2. ORM 与 sqlalchemy 模块