sqlalchemy一些常见概念

metadata

官方文档解释

The MetaData is a registry which includes the ability to emit a limited set of schema generation commands to the database. As our SQLite database does not actually have a users table present, we can use MetaData to issue CREATE TABLE statements to the database for all tables that don’t yet exist. Below, we call the MetaData.create_all() method, passing in our Engine as a source of database connectivity

metadata是一个注册器，里面包含一些数据库的模式生成命令，可以调用metadata发起create table命令来创建这个表

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mapper

sqlalchemy中创建一个继承declarative_base的类的本质是，创建一个class的同时生成一个同名的Table对象，并且使用一个mapper对象将两个对象映射在一起。

class MyClass(Base):
    __tablename__ = 'my_table'
    id = Column(Integer, primary_key=True)
    type = Column(String(50))
    alt = Column("some_alt", Integer)

    __mapper_args__ = {
        'polymorphic_on' : type
    }

上述代码跟下面效果是一样的

my_table = Table("my_table", metadata,
    Column('id', Integer, primary_key=True),
    Column('type', String(50)),
    Column("some_alt", Integer)
)

class MyClass(object):
    pass

mapper(MyClass, my_table,
    polymorphic_on=my_table.c.type,
    properties={
        'alt':my_table.c.some_alt
    })

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对象状态转换

值得一提的是，当class被add到session中，但是没有commit，此时这个class并没有被写入数据库，但是此时进行query查询这个class是可以被查询出来的，因为当执行query时，执行了flush操作，调用了sql语句，这个class对应的row被生成。

from sqlalchemy import Column, String, Integer, ForeignKey, create_engine
from sqlalchemy.orm import sessionmaker, relationship, backref
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
    __tablename__ = "user"
    id = Column(Integer, primary_key = True)
    name = Column(String(20))

engine = create_engine('mysql+mysqlconnector://root:Gzm20125@localhost:3306/test')
DBSession = sessionmaker(bind = engine)

Base.metadata.drop_all(engine)
Base.metadata.create_all(engine)
# 创建session对象:

session = DBSession()

u = User(id = 1, name = "gzm")
session.add(u)
print(session.query(User).filter_by(id = 1).first())

输出