sqlalchemy学习-- 重要参数

Base = declarative_base
基类：
1.存储表
2.建立class-Table 的映射关系
engine = create_engine('mysql://root:root@localhost:3307/test', echo=False)
连接数据库

Session = sessionmaker(bind=engine)
sess = Session()

会话
1.保存上下文信息
2.容器，存储对对象的操作。

metadata
所有Base的子类,共享这个metadata

1.所有Table存储在此metadata中
2.共享此metadata的属性和方法。

mapper
将class -- Table 进行映射

Column 重要参数
1.name  字段名（可选，该参数优先级高于变量名）
2.type_ 字段类型，注意：MySQL中，String类型必须指定长度
3.primary_key 如该参数为True，那么该字段为主键（False）
4.unique 如该参数如果为True，相应字段具有唯一性（False）
5.index 如该参数为True，相应字段为索引字段（False）
6.nullable 如该参数为False，该字段不允为空（True）
7.ForeignKey 通过该参数指定外键
8.default 指定该字段的默认值

engine 特性及重要参数
1.Engine是数据库核心接口
2.Lazy Connecting，此时并未真正建立其和数据库的连接，直到：engine.execute(),engine.connect()等方法被调用才会真正建立连接
3.pool_size = 5; 连接池的大小，默认为 5
4.max_overflow = 10; 连接池可溢出最大数量 默认为10 [不放入连接池]
5.echo = False;  打印SQL语句，调用logging模块,默认为False
6.encoding = 'utf-8'; 编码方式，默认为 utf8
7.pool_recycle = -1 连接回收时间 -1，永不回收（推荐设置3600即1h）
	注意： MySQL连接的默认断开时间是 8小时
8.pool_timeout=30 尝试从连接池中获取连接的超时时间

反向映射（即将数据库中的表映射成程序中的表对象）

Base.metadata.reflect(engine)
print(Base.metadata.tables)

print(Student.__table__)    # 查看类对应的表
print(Student.__mapper__)  # 查看类对应的mapper函数

engine = create_engine("...")
Session = sessionmaker(bind=engine)

# new session.   no connections are in use.
session = Session()
try:
    # first query.  a Connection is acquired
    # from the Engine, and a Transaction
    # started.
    item1 = session.query(Item).get(1)

    # second query.  the same Connection/Transaction
    # are used.
    item2 = session.query(Item).get(2)

    # pending changes are created.
    item1.foo = 'bar'
    item2.bar = 'foo'

    # commit.  The pending changes above
    # are flushed via flush(), the Transaction
    # is committed, the Connection object closed
    # and discarded, the underlying DBAPI connection
    # returned to the connection pool.
    session.commit()
except:
    # on rollback, the same closure of state
    # as that of commit proceeds.
    session.rollback()
    raise
finally:
    # close the Session.  This will expunge any remaining
    # objects as well as reset any existing SessionTransaction
    # state.  Neither of these steps are usually essential.
    # However, if the commit() or rollback() itself experienced
    # an unanticipated internal failure (such as due to a mis-behaved
    # user-defined event handler), .close() will ensure that
    # invalid state is removed.
    session.close()

关于数据库 字段为时间戳问题的探究：

/*create_time 自动保存创建时间；modify_time 自动保存修改时间*/
create table teacher(id int not null auto_increment primary key, name varchar(30),create_time TIMESTAMP default CURRENT_TIMESTAMP not null,modify_time TIMESTAMP default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP not null)ENGINE=InnoDB DEFAULT CHARSET=utf8;

show create table students; # 查看创建数据表的命令

import datetime
import time
from sqlalchemy import create_engine, Column, Integer, String, TIMESTAMP, text
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

engine = create_engine("mysql+pymysql://root:root@localhost/sqla?charset=utf8")

class Student(Base):
    __tablename__ = "students"
    __table_args__ = {
        # "mysql_engine":"MyISAM",
        "mysql_charset":"utf8"
    }  # show create table students 可以查看建表语句;默认是Innodb,lating-1.如果想显示中文需要修改指定建表的类型,同时,engine也要指定编码格式
    id = Column(Integer,primary_key=True)
    name = Column(String(30))
    # first  update_time 必须在上面,
    update_time = Column(TIMESTAMP,nullable=False)
    create_time = Column(TIMESTAMP,nullable=False,server_default=text("current_timestamp"))
    # create_time = Column(TIMESTAMP(True),nullable=False,server_default=text("now()"))
    # create_time = Column(TIMESTAMP(True),default=datetime.datetime.now()) # show create table students 会发现create_time 字段默认是null而不是current_timestamp,虽然可以正常使用，但是不推荐
    # second 颠倒 update_time 和 create_time 两个字段的顺序
    # create_time = Column(TIMESTAMP(True),nullable=False,server_default=text("current_timestamp"))
    # create_time = Column(TIMESTAMP(True),nullable=False,server_default=text("now()"))
    # create_time = Column(TIMESTAMP(True),nullable=False,default=datetime.datetime.now())   # error 会有 on update current_timestamp语句
    # update_time = Column(TIMESTAMP(True),nullable=False,server_default=text("current_timestamp on update current_timestamp"))
    # update_time = Column(TIMESTAMP(True),nullable=False,server_default=text("now() on update current_timestamp"))
    # third error  # 该例证明了直接颠倒两个字段会失败的.
    # create_time = Column(TIMESTAMP(True),nullable=False,server_default=text("current_timestamp"))
    # update_time = Column(TIMESTAMP(True),nullable=False)


Base.metadata.drop_all(engine)
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
sess = Session()

zhangsan = Student(name='张三')
sess.add(zhangsan)
sess.commit()
time.sleep(10)
zhangsan_1 = sess.query(Student).get(1)
zhangsan_1.name = 'lisi'
sess.commit()
wangwu = Student(name='wangwu')
sess.add(wangwu)
sess.commit()