Python廖雪峰实战Day3编写ORM思路整理

Python教程链接:Day 3 – 编写ORM

ORM

ORM(Object Relational Mapping)即对象关系映射,通过代码描述程序中对象和数据库对应的元数据,将对象持久化到数据库中。

创建连接池

创建链接池的作用是每个HTTP来请求数据库的时候,都能从连接池直接过去数据库的连接,而不需要每次都打开关闭数据库。

# 通过关键字参数**kw接受连接数据库需要的对应参数来创建连接池
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'],
        # 这里是utf8,而不是utf-8,别写错了
        charset=kw.get('charset', 'utf8'),
        autocommit=kw.get('autocommit', True),
        maxsize=kw.get('maxsize', 10),
        minsize=kw.get('minsize', 1),
        loop=loop
    )

对应的数据库操作语句的封装

Select

Select语句与UpdateInsertDelete语句分开进行封装,因为他返回的是所查询的数据库对应记录,而UpdateInsertDelete语句返回的是所影响的行数。

# 传入SQL语句,参数,大小可选
async def select(sql, args, size=None):
    log(sql)
    global __pool  # 获取全局的连接池__pool
    async with __pool.get() as conn:  # 打开连接池
        async with conn.cursor(aiomysql.DictCursor) as cur:  # 创建游标,DictCursor的作用是使查询返回的结果为字典格式
            await cur.execute(sql.replace('?', '%s'), args or ())  # 执行SQL语句,将SQL语句的'?'占位符替换成MySQL的'%s'占位符
            if size:  # 如果有传入size,则返回对应个数的结果,size为None则返回全部
                rs = await cur.fetchmany(size)
            else:
                rs = await cur.fetchall()
        logging.info('rows returned: %s' % len(rs))
        return rs

Update、Insert、Delete

这三种SQL语句的执行所需要的参数都相同,并且只返回一个整数表示所影响的行数。

# 传入SQL语句,参数,默认自动提交事务
async def execute(sql, args, autocommit=True):
    log(sql)
    async with __pool.get() as coon:  # 打开连接池
        if not autocommit:  # 如果autocommit为False,conn.begin()开始事务
            await coon.begin()
        try:  # 无论是否自动提交事务,都执行try中的代码
            async with coon.cursor(aiomysql.DictCursor) as cur:  # 创建游标,DictCursor的作用是使查询返回的结果为字典格式
                await cur.execute(sql.replace('?', '%s'), args)
                affected = cur.rowcount  # 通过rowcount得到SQL语句影响的行数
            if not autocommit:
                await coon.commit()
        except BaseException as e:  # 处理出错情况
            if not autocommit:
                await coon.roolback()  # 回滚操作
            raise
        return affected

Field基类及其子类

定义一个Field类型和其子类对应数据库中不同的类型,StringIntegerBooleanFloatText等。

class Field(object):

    def __init__(self, name, column_type, primary_key, default):
        self.name = name  # 字段名
        self.column_type = column_type  # 列类型
        self.primary_key = primary_key  # 主键
        self.default = default  # 默认值

    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)

元类

通过元类ModelMetaclass可以将具体的子类映射信息读取出来。

class ModelMetaclass(type):
    # 调用__init__方法前会调用__new__方法
    def __new__(cls, name, bases, attrs):  # cls:当前准备创建的类的对象,name:类的名称,bases:类继承的父类集合,attrs:类的方法集合
        # 排除Model类本身
        if name == 'Model':
            return type.__new__(cls, name, bases, attrs)
        # 获取table名称,如果未设置,tableName就是类的名字
        tableName = attrs.get('__table__', None) or name
        logging.info('found model: %s (table: %s)' % (name, tableName))
        # 获取所有的Field和主键名
        mappings = dict()
        fields = []
        primaryKey = None
        # key是列名,value是field的子类
        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 RuntimeError('Duplicate primary key for field: %s' % k)
                    primaryKey = k
                else:
                    fields.append(k)
        if not primaryKey:
            raise RuntimeError('Primary key not found.')
        # 删除类属性
        for k in mappings.keys():
            attrs.pop(k)
        # 保存除主键外的属性名为``(运算出字符串)列表形式
        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
        # 构造默认的SELECT, INSERT, UPDATE和DELETE语句
        # 反引号和repr()函数的功能一致
        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['__delete__'] = 'delete from `%s` where `%s`=?' % (tableName, primaryKey)
        return type.__new__(cls, name, bases, attrs)

基类Model

继承自Model的类,会自动通过ModelMetaclass扫描映射关系,并存储到自身的类属性中__table____mappings__等。

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):
        # 返回对象的属性,如果没有对应属性,则会调用__getattr__
        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)))
                # 将默认值设置进行
                setattr(self, key, value)
        return value

    # 类方法第一个参数为cls,而实例方法第一个参数为self
    @classmethod
    async def findAll(cls, where=None, args=None, **kw):
        """find object 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('?', '?')
                # extend接受一个iterable参数
                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"""
        # 将列名重命名为_num_
        sql = ['select %s _num_ from `%s`' % (selectField, cls.__table__)]
        if where:
            sql.append('where')
            sql.append(where)
            # 限制结果数为1
        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 primarykey"""
        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):
        # 获取所有value
        args = list(map(self.getValueOrDefault, self.__fields__))
        args.append(self.getValueOrDefault(self.__primary_key__))
        rows = await execute(self.__insert__, args)
        if rows != 1:
            logging.warning('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.warning('faild 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.warning('faild to remove by primary key: affected rows: %s' % rows)

注意点

Python采用的是代码缩进来区分代码块的,而不是其他语言中的{}。所以写代码的时候要格外注意,刚开始用Python经常会因为这个问题导致程序不按预期执行。
博主因为粗心写错了一个for循环内的return缩进,调试了很久才醒悟。

    原文作者:酱油白切鸡
    原文地址: https://www.jianshu.com/p/da1e684d92b5
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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