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Python中操作关系数据库最直接的就是用DB-API了,流程一般是:连接、执行SQL语句、提交、断开。以MySQL为例,下面是各步骤的代码示例:
首先是连接:
- % python
- >>> import MySQLdb
- >>> conn = MySQLdb.connect(host='localhost', user='root', passwd='python')
% python>>> import MySQLdb>>> conn = MySQLdb.connect(host='localhost', user='root', passwd='python')
接着便可以执行语句了,但在执行SQL语句前要先获取指针:
- >>> curs = conn.cursor( )
- >>> curs.execute('create database peopledb')
1L- >>> curs.execute('use peopledb')
0L- >>> tblcmd = 'create table people (name char(30), job char(10), pay int(4))'
- >>> curs.execute(tblcmd)
0L
>>> curs = conn.cursor( )>>> curs.execute('create database peopledb')1L>>> curs.execute('use peopledb')0L>>> tblcmd = 'create table people (name char(30), job char(10), pay int(4))'>>> curs.execute(tblcmd)0L
添加数据:
- >>> curs.execute('insert people values (%s, %s, %s)', ('Bob', 'dev', 5000))
1L- >>> curs.executemany('insert people values (%s, %s, %s)',
- ... [ ('Sue', 'mus', '70000'),
- ... ('Ann', 'mus', '60000')])
2L- >>> conn.commit( )
>>> curs.execute('insert people values (%s, %s, %s)', ('Bob', 'dev', 5000))1L>>> curs.executemany('insert people values (%s, %s, %s)',... [ ('Sue', 'mus', '70000'),... ('Ann', 'mus', '60000')])2L>>> conn.commit( )
执行查询:
- >>> curs.execute('select * from people')
6L- >>> curs.fetchall( )
- (('Bob', 'dev', 5000L), ('Sue', 'mus', 70000L), ('Ann', 'mus', 60000L), ('Tom',
'mgr', 100000L))
>>> curs.execute('select * from people')6L>>> curs.fetchall( )(('Bob', 'dev', 5000L), ('Sue', 'mus', 70000L), ('Ann', 'mus', 60000L), ('Tom','mgr', 100000L))
执行完数据库操作记得断开连接:
- conn.close( ) # close, _ _del_ _ call rollback if changes not committed yet
conn.close( ) # close, _ _del_ _ call rollback if changes not committed yet
如果数据结构不是很复杂,配合Python强大的列表解析能力,不用ORM框架也是很方便的;或者自己封装对象映射也不是很难。
如果使用了Django框架,可以使用它自带的ORM工具来操作数据库。首先当然是编写实体类(或者叫模型)了:
- from django.db import models
class Musician(models.Model): - first_name = models.CharField(max_length=50)
- last_name = models.CharField(max_length=50)
- instrument = models.CharField(max_length=100)
class Album(models.Model): - artist = models.ForeignKey(Musician)
- name = models.CharField(max_length=100)
- release_date = models.DateField()
- num_stars = models.IntegerField()
from django.db import modelsclass Musician(models.Model): first_name = models.CharField(max_length=50) last_name = models.CharField(max_length=50) instrument = models.CharField(max_length=100)class Album(models.Model): artist = models.ForeignKey(Musician) name = models.CharField(max_length=100) release_date = models.DateField() num_stars = models.IntegerField()
Python的代码已经很清楚了,类对应表,成员变量对应表的列,列属性由models.XXXField(...)定义。如果实体类没有显式定义主键,Django会默认加上一句:
- id = models.AutoField(primary_key=True)
id = models.AutoField(primary_key=True)
Django里可以这样定义枚举型数据:
- class Person(models.Model):
- GENDER_CHOICES = (
- (u'M', u'Male'),
- (u'F', u'Female'),
- )
- name = models.CharField(max_length=60)
- gender = models.CharField(max_length=2, choices=GENDER_CHOICES)
class Person(models.Model): GENDER_CHOICES = ( (u'M', u'Male'), (u'F', u'Female'), ) name = models.CharField(max_length=60) gender = models.CharField(max_length=2, choices=GENDER_CHOICES)
对于关联关系,在做列的映射定义时可以这么写:
- poll = models.ForeignKey(Poll)
- sites = models.ManyToManyField(Site)
- place = models.OneToOneField(Place")
poll = models.ForeignKey(Poll)sites = models.ManyToManyField(Site)place = models.OneToOneField(Place")
在Django里定义关联关系还有更多功能,详细的还是看官方文档吧~
Django的Model基类中已经定义了基本的数据库操作,因为所有的实体类都是继承自Model类,所以也就有了这些操作。例如新建并保存一个person只需要这么做:
- >>> p = Person(name="Fred Flinstone", gender="M")
- >>> p.save()
>>> p = Person(name="Fred Flinstone", gender="M")>>> p.save()
Django会通过查询对象的主键是否存在来决定该UPDATE还是INSERT,当然你也可以强制框架执行某种操作。如果你不满意框架自带的方法,可以重写它:
- class Blog(models.Model):
- name = models.CharField(max_length=100)
- tagline = models.TextField()
def save(self, *args, **kwargs): - do_something()
- super(Blog, self).save(*args, **kwargs) # Call the "real" save() method.
- do_something_else()
class Blog(models.Model): name = models.CharField(max_length=100) tagline = models.TextField() def save(self, *args, **kwargs): do_something() super(Blog, self).save(*args, **kwargs) # Call the "real" save() method. do_something_else()
发现没,Django里存取数据不需要那种session,最讨厌Hibernate里的session了,总是报“Session Closed”错误……
Python还有一个独立的ORM框架——SQLAlchemy。功能更强大,支持的数据库也比Django自带的ORM工具要多。它有两种建立实体类的方法。
一种是分开定义,再将表定义和类定义映射起来。首先是建立表的定义:
- >>> from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey
- >>> metadata = MetaData()
- >>> users_table = Table('users', metadata,
- ... Column('id', Integer, Sequence('user_id_seq'), primary_key=True),
- ... Column('name', String(50)),
- ... Column('fullname', String(50)),
- ... Column('password', String(12))
- ... )
>>> from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey>>> metadata = MetaData()>>> users_table = Table('users', metadata,... Column('id', Integer, Sequence('user_id_seq'), primary_key=True),... Column('name', String(50)),... Column('fullname', String(50)),... Column('password', String(12))... )
接着定义实体类:
- >>> class User(object):
- ... def __init__(self, name, fullname, password):
- ... self.name = name
- ... self.fullname = fullname
- ... self.password = password
>>> class User(object):... def __init__(self, name, fullname, password):... self.name = name... self.fullname = fullname... self.password = password
这还没完,还要把他们映射起来:
- >>> from sqlalchemy.orm import mapper
- >>> mapper(User, users_table)
>>> from sqlalchemy.orm import mapper>>> mapper(User, users_table)
这样的过程有点像Hibernate里将XML的Map文件和实体类的映射。Hibernate中还可以方便的直接用注释在实体类中完成与表的映射,当然SQLAlchemy也有直接的方法:
- >>> from sqlalchemy.ext.declarative import declarative_base
- >>> Base = declarative_base()
- >>> class User(Base):
- ... __tablename__ = 'users'
- ...
- ... id = Column(Integer, primary_key=True)
- ... name = Column(String)
- ... fullname = Column(String)
- ... password = Column(String)
>>> from sqlalchemy.ext.declarative import declarative_base>>> Base = declarative_base()>>> class User(Base):... __tablename__ = 'users'...... id = Column(Integer, primary_key=True)... name = Column(String)... fullname = Column(String)... password = Column(String)
作为一个独立的ORM框架,实体类的存取当然就不会像Django那样集成的那么完美了,SQLAlchemy里存取数据也是要Session的:
- >>> from sqlalchemy.orm import sessionmaker
- >>> Session = sessionmaker(bind=engine)
>>> from sqlalchemy.orm import sessionmaker>>> Session = sessionmaker(bind=engine)
这里的engine对象需要这样建立:
- >>> from sqlalchemy import create_engine
- >>> engine = create_engine('<SPAN style="LINE-HEIGHT: 20px; FONT-FAMILY: monospace; WHITE-SPACE: normal; COLOR: #333333">dialect+driver://user:password@host/dbname[?key=value..]</SPAN>', echo=True)
>>> from sqlalchemy import create_engine>>> engine = create_engine('dialect+driver://user:password@host/dbname[?key=value..]', echo=True)
对于存取操作,如果是保存就这么写:
- >>> ed_user = User('ed', 'Ed Jones', 'edspassword')
- >>> session.add(ed_user)
>>> ed_user = User('ed', 'Ed Jones', 'edspassword')>>> session.add(ed_user)
如果要查询,就是类似的这种形式:
- >>> our_user = session.query(User).filter_by(name='ed').first()
>>> our_user = session.query(User).filter_by(name='ed').first()
执行完一些数据操作,必要的时候要提交或是回滚:
- >>> session.rollback()
- 或者
- >>> session.commit()
>>> session.rollback()或者>>> session.commit()
SQLAlchemy框架还有一个衍生产品——Elixir,在SQLAlchemy的基础上对其映射方式做了些封装,使得实体类的定义有点类似Django中的定义方式。
以上便是这两天对Python中数据存储的一些学习记录。话说Django的ORM与它的其他模块结合的很紧密,不好单独使用;SQLAlchemy虽然强大,但风格不太喜欢,所以下一步打算深入两个ORM框架的代码,看看他们是怎么实现的。一方面好抉择用哪一个,另外也可以看看在自己的应用中能否自己做一个简单的ORM。 |
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