Python实战-编写Web App-Day3-编写ORM

Python实战 木人张 7个月前 (04-11) 80次浏览 0个评论 扫描二维码
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什么是ORM?对象关系映射(Object Relational Mapping,简称ORM)是通过使用描述对象和数据库之间映射的元数据,将面向对象语言程序中的对象自动持久化到关系数据库中。

一个webapp中,所有数据都储存在数据库中,这里用的mysql。webapp中很多地方要访问数据库,所以需要一个统一的函数,把mysql的操作封装起来。
由于Web框架使用了基于asyncio的aiohttp,这是基于协程的异步模型。异步编程的一个原则:一旦决定异步,系统每一层都是异步。

创建连接池

创建一个全局连接池,每个HTTP请求都从连接池中直接获取数据库连接。使用连接池的好处是不必频繁地打开和关闭数据库连接,而是能复用就尽量复用。连接池由全局变量__pool存储,缺省情况下将编码设置为utf8,自动提交事务:

@asyncio.coroutine
def create_pool(loop, **kw):
    logging.info('create database connection pool...')
    global __pool
    __pool = yield from aiomysql.create_pool(
        host=kw.get('host','localhost'),
        port=kw.get('port', 3306),
        user=kw['user'],
        password=kw['password'],
        db=kw['db'],
        charset=kw.get('charset','utf-8'),
        autocommit=kw.get('autocommit', True),
        maxsize=kw.get('maxsize',10),
        minsize=ke.get('minsize',1),
        loop=loop
    )

SELECT

要执行mysql的SELECT语句,我们用select函数执行,需要传入SQL语句和SQL参数:

@asyncio.coroutine
def select(sql, args, size=None):
    log(sql, args)
    global __pool
    with (yield from __pool) as conn:
        cur = yield from conn.cursor(aiomysql.DictCursor)
        yield from cur.execute(sql.replace('?', '%s'), args or ())
        if size:
            rs = yield from cur.fetchmany(size)
        else:
            rs = yield from cur.fetchall()
        yield from cur.close()
        logging.info('rows returned: %s' % len(rs))
        return rs

Insert, Update, Delete

要执行INSERT、UPDATE、DELETE语句,可以定义一个通用的execute()函数,因为这3种SQL的执行都需要相同的参数,以及返回一个整数表示影响的行数:

@asyncio.coroutine
def execute(sql, args):
    log(sql)
    with (yield from __pool) as conn:
        try:
            cur = yield from conn.cursor()
            yield from cur.execute(sql.replace('?', '%s'), args)
            affected = cur.rowcount
            yield from cur.clous()
        except BaseException as e:
            raise
        return affected

execute()函数和select()函数所不同的是,cursor对象不返回结果集,而是通过rowcount返回结果数。

ORM

有了基本的select()和execute()函数,我们就可以开始编写一个简单的ORM了。设计ORM需要从上层调用者角度来设计。我们先考虑如何定义一个User对象,然后把数据库表users和它关联起来:

from orm import Model, StringField, IntegerField

class User(Model):
    __table__ = 'users'
    
    id = IntegerField(primary_ken=True)
    name = StringField()

注意到定义在User类中的__table__、id和name是类的属性,不是实例的属性。所以,在类级别上定义的属性用来描述User对象和表的映射关系,而实例属性必须通过__init__()方法去初始化,所以两者互不干扰:

# 创建实例:
user = User(id=1, name='Woodman')
# 存入数据库:
user.insert()
# 查询所有User对象:
users = User.findAll()

定义Model

首先要定义的是所有ORM映射的基类Model:

class Model(dict, metaclass=ModelMetaclass):

    def __init__(self, **kw):
        super(Model, self).__init__(**kw)

    def __getattr__(self, key):
        try:
            return self[key]
        except KeyError:
            raise AttributeError(r" 'Model' object has no attribute '%s'" % key)

    def __setattr__(self, key, value):
        self[key] = value

    def getValue(self, key):
        return getattr(self, key, None)

    def getValueOrDefault(self, key):
        value = getattr(self, key ,None)
        if value is None:
            field = self.__mappings__[key]
            if field.default is not None:
                value = field.default() if callable(field.default) else field.default
                logging.debug('using default value for %s: %s' % (key, str(value)))
                setattr(self,key,value)
            return value

Model从dict继承,所以具备所有dict的功能,同时又实现了特殊方法__getattr__()和__setattr__(),因此又可以像引用普通字段那样写:

>>> user['id']
123
>>> user.id
123

以及Field和各种Field子类:

calss Field(object):

    def __init__(self, name, column_type, primary_key, default):
        self.name = name
        self.column_type = column_type
        self.primary_key = primary_key
        self.default = default

    def __str__(self):
        return '<%s, %s:%s>' % (self.__class__.__name__,self.column_type,self.name)

映射varchar的StringField:

class StringField(Field):
    def __init__(self, name=None, primary_key=False, default=None, ddl='varchar(100)'):
        super().__init__(name,ddl,primary_key,default)

Model只是一个基类,如何将具体的子类如User的映射信息读取出来呢?答案就是通过metaclass:ModelMetaclass:

class ModelMetaclass(type):

    def __new__(cls, name, bases, attrs):
        #排除Model类本身
        if name =='Model':
            return type.__new__(cls, name, bases, attrs)
        #获取table名称
        tableName = attrs.get('__tabale__', None) or name
        logging.info('found model: %s (table: %s)' % (name, tableName))
        #获取所有Field和主键名
        mappings = dict()
        fields = []
        primaryKey = None
        for k, v in attrs.iterms():
            if isinstance(v, Field):
                logging.info('found mapping: %s ==> %s' % (k, v))
                mappings[k]= v
                if v.primary_key:
                    #找到主键:
                    if primaryKey:
                        raise RuntimeError('Duplicate primary key for field: %s' % k)
                    primaryKey = k
                else:
                    fields.append(k)
        if not primaryKey:
            raise RuntimeError('Primary key bot found.')
        for k in mappings.keys():
            attrs.pop(k)
        escaped_fields = list(map(lambda f: '`%s`' % f, fields))
        attrs['__mappings__'] = mappings
        attrs['__table__'] = tableName
        attrs['__primary_key__'] = primaryKey
        attrs['__fields__'] = fields
        attrs['__select__'] = 'select `%s`, %s from `%s`' % (primaryKey, ', '.join(escaped_fields), tableName)
        attrs['__insert__'] = 'insert into `%s` (%s, `%s`) values (%s)' % (tableName, ', '.join(escaped_fields), primaryKey, create_args_string(len(escaped_fields) + 1))
        attrs['__update__'] = 'update `%s` set %s where `%s`=?' % (tableName, ', '.join(map(lambda f: '`%s`=?' % (mappings.get(f).name or f), fields)), primaryKey)
        attrs['__delete__'] = 'delete from `%s` where `%s`=?' % (tableName, primaryKey)
        return type.__new__(cls, name, bases, attrs)

这样,任何继承自Model的类(比如User),会自动通过ModelMetaclass扫描映射关系,并存储到自身的类属性如__table__、__mappings__中。

然后,我们往Model类添加class方法,就可以让所有子类调用class方法:

class Model(dict):

    ...

    @classmethod
    @asyncio.coroutine
    def find(cls, pk):
        ' find object by primary key. '
        rs = yield from select('%s where `%s`=?' % (cls.__select__, cls.__primary_key__), [pk], 1)
        if len(rs) == 0:
            return None
        return cls(**rs[0])

User类现在就可以通过类方法实现主键查找:

user = yield from User.find('123')

往Model类添加实例方法,就可以让所有子类调用实例方法:

class Model(dict):

    ...

    @asyncio.coroutine
    def save(self):
        args = list(map(self.getValueOrDefault, self.__fields__))
        args.append(self.getValueOrDefault(self.__primary_key__))
        rows = yield from execute(self.__insert__, args)
        if rows != 1:
            logging.warn('failed to insert record: affected rows: %s' % rows)

这样,就可以把一个User实例存入数据库:

user = User(id=1, name='WoodmanZhang')
yield from user.save()

最后一步是完善ORM,对于查找,我们可以实现以下方法:

findAll() – 根据WHERE条件查找;
findNumber() – 根据WHERE条件查找,但返回的是整数,适用于select count(*)类型的SQL。

以及update()和remove()方法。
所有这些方法都必须用@asyncio.coroutine装饰,变成一个协程。
调用时需要特别注意:
user.save()
没有任何效果,因为调用save()仅仅是创建了一个协程,并没有执行它。一定要用:
yield from user.save()
才真正执行了INSERT操作。

完整代码如下:

# -*- coding: UTF-8 -*-
# Filename : orm.py
# author by : www.woodmanzhang.com
__author__ = 'WoodmanZhang.com'
import asyncio, logging

import aiomysql

def log(sql, args=()):
    logging.info('SQL: %s' % sql)

async def create_pool(loop, **kw):
    logging.info('create database connection pool...')
    global __pool
    __pool = await aiomysql.create_pool(
        host=kw.get('host', 'localhost'),
        port=kw.get('port', 3306),
        user=kw['user'],
        password=kw['password'],
        db=kw['db'],
        charset=kw.get('charset', 'utf8'),
        autocommit=kw.get('autocommit', True),
        maxsize=kw.get('maxsize', 10),
        minsize=kw.get('minsize', 1),
        loop=loop
    )

async def select(sql, args, size=None):
    log(sql, args)
    global __pool
    async with __pool.get() as conn:
        async with conn.cursor(aiomysql.DictCursor) as cur:
            await cur.execute(sql.replace('?', '%s'), args or ())
            if size:
                rs = await cur.fetchmany(size)
            else:
                rs = await cur.fetchall()
        logging.info('rows returned: %s' % len(rs))
        return rs

async def execute(sql, args, autocommit=True):
    log(sql)
    async with __pool.get() as conn:
        if not autocommit:
            await conn.begin()
        try:
            async with conn.cursor(aiomysql.DictCursor) as cur:
                await cur.execute(sql.replace('?', '%s'), args)
                affected = cur.rowcount
            if not autocommit:
                await conn.commit()
        except BaseException as e:
            if not autocommit:
                await conn.rollback()
            raise
        return affected

def create_args_string(num):
    L = []
    for n in range(num):
        L.append('?')
    return ', '.join(L)

class Field(object):

    def __init__(self, name, column_type, primary_key, default):
        self.name = name
        self.column_type = column_type
        self.primary_key = primary_key
        self.default = default

    def __str__(self):
        return '<%s, %s:%s>' % (self.__class__.__name__, self.column_type, self.name)

class StringField(Field):

    def __init__(self, name=None, primary_key=False, default=None, ddl='varchar(100)'):
        super().__init__(name, ddl, primary_key, default)

class BooleanField(Field):

    def __init__(self, name=None, default=False):
        super().__init__(name, 'boolean', False, default)

class IntegerField(Field):

    def __init__(self, name=None, primary_key=False, default=0):
        super().__init__(name, 'bigint', primary_key, default)

class FloatField(Field):

    def __init__(self, name=None, primary_key=False, default=0.0):
        super().__init__(name, 'real', primary_key, default)

class TextField(Field):

    def __init__(self, name=None, default=None):
        super().__init__(name, 'text', False, default)

class ModelMetaclass(type):

    def __new__(cls, name, bases, attrs):
        if name=='Model':
            return type.__new__(cls, name, bases, attrs)
        tableName = attrs.get('__table__', None) or name
        logging.info('found model: %s (table: %s)' % (name, tableName))
        mappings = dict()
        fields = []
        primaryKey = None
        for k, v in attrs.items():
            if isinstance(v, Field):
                logging.info('  found mapping: %s ==> %s' % (k, v))
                mappings[k] = v
                if v.primary_key:
                    # 找到主键:
                    if primaryKey:
                        raise StandardError('Duplicate primary key for field: %s' % k)
                    primaryKey = k
                else:
                    fields.append(k)
        if not primaryKey:
            raise StandardError('Primary key not found.')
        for k in mappings.keys():
            attrs.pop(k)
        escaped_fields = list(map(lambda f: '`%s`' % f, fields))
        attrs['__mappings__'] = mappings # 保存属性和列的映射关系
        attrs['__table__'] = tableName
        attrs['__primary_key__'] = primaryKey # 主键属性名
        attrs['__fields__'] = fields # 除主键外的属性名
        attrs['__select__'] = 'select `%s`, %s from `%s`' % (primaryKey, ', '.join(escaped_fields), tableName)
        attrs['__insert__'] = 'insert into `%s` (%s, `%s`) values (%s)' % (tableName, ', '.join(escaped_fields), primaryKey, create_args_string(len(escaped_fields) + 1))
        attrs['__update__'] = 'update `%s` set %s where `%s`=?' % (tableName, ', '.join(map(lambda f: '`%s`=?' % (mappings.get(f).name or f), fields)), primaryKey)
        attrs['__delete__'] = 'delete from `%s` where `%s`=?' % (tableName, primaryKey)
        return type.__new__(cls, name, bases, attrs)

class Model(dict, metaclass=ModelMetaclass):

    def __init__(self, **kw):
        super(Model, self).__init__(**kw)

    def __getattr__(self, key):
        try:
            return self[key]
        except KeyError:
            raise AttributeError(r"'Model' object has no attribute '%s'" % key)

    def __setattr__(self, key, value):
        self[key] = value

    def getValue(self, key):
        return getattr(self, key, None)

    def getValueOrDefault(self, key):
        value = getattr(self, key, None)
        if value is None:
            field = self.__mappings__[key]
            if field.default is not None:
                value = field.default() if callable(field.default) else field.default
                logging.debug('using default value for %s: %s' % (key, str(value)))
                setattr(self, key, value)
        return value

    @classmethod
    async def findAll(cls, where=None, args=None, **kw):
        ' find objects by where clause. '
        sql = [cls.__select__]
        if where:
            sql.append('where')
            sql.append(where)
        if args is None:
            args = []
        orderBy = kw.get('orderBy', None)
        if orderBy:
            sql.append('order by')
            sql.append(orderBy)
        limit = kw.get('limit', None)
        if limit is not None:
            sql.append('limit')
            if isinstance(limit, int):
                sql.append('?')
                args.append(limit)
            elif isinstance(limit, tuple) and len(limit) == 2:
                sql.append('?, ?')
                args.extend(limit)
            else:
                raise ValueError('Invalid limit value: %s' % str(limit))
        rs = await select(' '.join(sql), args)
        return [cls(**r) for r in rs]

    @classmethod
    async def findNumber(cls, selectField, where=None, args=None):
        ' find number by select and where. '
        sql = ['select %s _num_ from `%s`' % (selectField, cls.__table__)]
        if where:
            sql.append('where')
            sql.append(where)
        rs = await select(' '.join(sql), args, 1)
        if len(rs) == 0:
            return None
        return rs[0]['_num_']

    @classmethod
    async def find(cls, pk):
        ' find object by primary key. '
        rs = await select('%s where `%s`=?' % (cls.__select__, cls.__primary_key__), [pk], 1)
        if len(rs) == 0:
            return None
        return cls(**rs[0])

    async def save(self):
        args = list(map(self.getValueOrDefault, self.__fields__))
        args.append(self.getValueOrDefault(self.__primary_key__))
        rows = await execute(self.__insert__, args)
        if rows != 1:
            logging.warn('failed to insert record: affected rows: %s' % rows)

    async def update(self):
        args = list(map(self.getValue, self.__fields__))
        args.append(self.getValue(self.__primary_key__))
        rows = await execute(self.__update__, args)
        if rows != 1:
            logging.warn('failed to update by primary key: affected rows: %s' % rows)

    async def remove(self):
        args = [self.getValue(self.__primary_key__)]
        rows = await execute(self.__delete__, args)
        if rows != 1:
            logging.warn('failed to remove by primary key: affected rows: %s' % rows)

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