ORM（Object Relational Mapping，对象关系映射)，是一种程序设计技术，用于实现面向对象编程语言里不同类型系统的数据之间的转换。从效果上来说，它其实创建了一个可在编程语言里使用的“虚拟对象数据库”。
上面是维基百科的解释，但是为什么要用ORM这种编程技术呢？
就这个实战作业来看：
博客——标题、摘要、内容、评论、作者、创作时间
评论——内容、评论人、评论文章、评论时间
用户——姓名、邮箱、口令、权限
上述信息，都需要有组织的存储在数据库中。数据库方面很简单，只需要维护三张表，如果想要加强各表间的联系，可以使用外键。但是，Python该如何组织这些信息呢？每篇博客有不同标题、摘要、内容...，每篇评论和用户信息也个不相同。像C这种面向过程的语言必须创建高级数据结构，而Python这种面向对象的语言真是有天然的优势，我们只需要把每篇博客、评论或者用户看作对象，使用属性表示其蕴含的信息。最后，我们还要解决一个问题：Python和数据库如何高效有组织的交换数据呢？数据库表是由一条条记录组成的，每条记录又包含不同字段。记录和字段，对象和属性...看起来两者关系类不类似？这就是我们的思路——
将数据库表的每条记录映射为对象，每条记录的字段和对象的属性相应；同时透过对象方法执行SQL命令。
我们编写的ORM框架就是实现上述想法。
1 #!/usr/bin/env python3 2 # -*- coding: utf-8 -*- 3 #Program: 4 # This is a ORM for MySQL. 5 #History: 6 #2017/06/29 smile First release 7 8 import logging 9 import asyncio 10 import aiomysql 11 12 def log(sql,args=()): 13 logging.info('SQL: %s' % sql) 14 15 #Close pool 16 async def destory_pool(): 17 global __pool 18 __pool.close() 19 await __pool.wait_closed() 20 21 #Create connect pool 22 #Parameter: host,port,user,password,db,charset,autocommit 23 # maxsize,minsize,loop 24 async def create_pool(loop,**kw): 25 logging.info('Create database connection pool...') 26 global __pool 27 __pool = await aiomysql.create_pool( 28 host = kw.get('host', 'localhost'), 29 port = kw.get('port', 3306), 30 user = kw['user'], 31 password = kw['password'], 32 db = kw['db'], 33 charset = kw.get('charset', 'utf8'), 34 autocommit = kw.get('autocommit', 'True'), 35 maxsize = kw.get('maxsize', 10), 36 minsize = kw.get('minsize', 1), 37 loop = loop 38 ) 39 40 #Package SELECT function that can execute SELECT command. 41 #Setup 1:acquire connection from connection pool. 42 #Setup 2:create a cursor to execute MySQL command. 43 #Setup 3:execute MySQL command with cursor. 44 #Setup 4:return query result. 45 async def select(sql,args,size=None): 46 log(sql,args) 47 global __pool 48 async with __pool.acquire() as conn: 49 async with conn.cursor(aiomysql.DictCursor) as cur: 50 await cur.execute(sql.replace('?','%s'),args or ()) 51 if size: 52 rs = await cur.fetchmany(size) 53 else: 54 rs = await cur.fetchall() 55 56 logging.info('rows returned: %s' % len(rs)) 57 return rs 58 59 #Package execute function that can execute INSERT,UPDATE and DELETE command 60 async def execute(sql,args,autocommit=True): 61 global __pool 62 #acquire connection from connection pool 63 async with __pool.acquire() as conn: 64 #如果MySQL禁止隐式提交，则标记事务开始 65 if not autocommit: 66 await conn.begin() 67 try: 68 #create cursor to execute MySQL command 69 async with conn.cursor(aiomysql.DictCursor) as cur: 70 await cur.execute(sql.replace('?','%s'),args or ()) 71 affectrow = cur.rowcount 72 #如果MySQL禁止隐式提交，手动提交事务 73 if not autocommit: 74 await cur.commit() 75 #如果事务处理出现错误，则回退 76 except BaseException as e: 77 await conn.rollback() 78 raise 79 80 #return number of affected rows 81 return affectrow 82 83 #Create placeholder with '?' 84 def create_args_string(num): 85 L = [] 86 for i in range(num): 87 L.append('?') 88 return ', '.join(L) 89 90 #A base class about Field 91 #描述字段的字段名，数据类型，键信息，默认值 92 class Field(object): 93 def __init__(self,name,column_type,primary_key,default): 94 self.name = name 95 self.column_type = column_type 96 self.primary_key = primary_key 97 self.default = default 98 99 def __str__(self): 100 return '<%s,%s:%s>' % (self.__class__.__name__,self.column_type,self.name) 101 102 #String Field 103 class StringField(Field): 104 def __init__(self,name=None,ddl='varchar(100)',default=None,primary_key=False): 105 super(StringField,self).__init__(name,ddl,primary_key,default) 106 107 #Bool Fileed 108 class BooleanField(Field): 109 def __init__(self,name=None,ddl='boolean',default=False,primary_key=False): 110 super(BooleanField,self).__init__(name,ddl,primary_key,default) 111 112 #Integer Field 113 class IntegerField(Field): 114 def __init__(self,name=None,ddl='bigint',default=None,primary_key=None): 115 super(IntegerField,self).__init__(name,ddl,primary_key,default) 116 117 #Float Field 118 class FloatField(Field): 119 def __init__(self,name=None,ddl='real',default=None,primary_key=None): 120 super(FloatField,self).__init__(name,ddl,primary_key,default) 121 122 #Text Field 123 class TextField(Field): 124 def __init__(self,name=None,ddl='text',default=None,primary_key=None): 125 super(TextField,self).__init__(name,ddl,primary_key,default) 126 127 #Meatclass about ORM 128 #作用： 129 #首先，拦截类的创建 130 #然后，修改类 131 #最后，返回修改后的类 132 class ModelMetaclass(type): 133 #采集应用元类的子类属性信息 134 #将采集的信息作为参数传入__new__方法 135 #应用__new__方法修改类 136 def __new__(cls,name,bases,attrs): 137 #不对Model类应用元类 138 if name == 'Model': 139 return type.__new__(cls,name,bases,attrs) 140 141 #获取数据库表名。若__table__为None,则取用类名 142 tablename = attrs.get('__table__',None) or name 143 logging.info('Found model: %s (table: %s)' % (name,tablename)) 144 145 #存储映射表类的属性（键-值） 146 mappings = dict() 147 #存储映射表类的非主键属性(仅键） 148 fields = [] 149 #主键对应字段 150 primarykey = None 151 for k,v in attrs.items(): 152 if isinstance(v,Field): 153 logging.info('Found mapping: %s ==> %s' % (k,v)) 154 mappings[k] = v 155 156 if v.primary_key: 157 logging.info('Found primary key') 158 if primarykey: 159 raise Exception('Duplicate primary key for field:%s' % k) 160 primarykey = k 161 else: 162 fields.append(k) 163 164 #如果没有主键抛出异常 165 if not primarykey: 166 raise Exception('Primary key not found') 167 168 #删除映射表类的属性，以便应用新的属性 169 for i in mappings.keys(): 170 attrs.pop(i) 171 172 #使用反单引号" ` "区别MySQL保留字，提高兼容性 173 escaped_fields = list(map(lambda f:'`%s`' % f,fields)) 174 175 #重写属性 176 attrs['__mappings__'] = mappings 177 attrs['__table__'] = tablename 178 attrs['__primary_key__'] = primarykey 179 attrs['__fields__'] = fields 180 attrs['__select__'] = 'SELECT `%s`, %s FROM `%s`' % (primarykey,','.join(escaped_fields),tablename) 181 attrs['__insert__'] = 'INSERT `%s` (%s,`%s`) VALUES (%s)' % (tablename,','.join(escaped_fields),primarykey,create_args_string(len(escaped_fields) + 1)) 182 attrs['__update__'] = 'UPDATE `%s` SET %s WHERE `%s` = ?' % (tablename,','.join(map(lambda f:'`%s` = ?' % (mappings.get(f).name or f),fields)),primarykey) 183 attrs['__delete__'] = 'DELETE FROM `%s` WHERE `%s` = ?' % (tablename,primarykey) 184 185 #返回修改后的类 186 return type.__new__(cls,name,bases,attrs) 187 188 #A base class about Model 189 #继承dict类特性 190 #附加方法： 191 # 以属性形式获取值 192 # 拦截私设属性 193 class Model(dict,metaclass=ModelMetaclass): 194 def __init__(self,**kw): 195 super(Model,self).__init__(**kw) 196 197 def __getattr__(self,key): 198 try: 199 return self[key] 200 except KeyError: 201 raise AttributeError(r"'Model' object has no attribute '%s'" % key) 202 203 def __setattr__(self,key,value): 204 self[key] = value 205 206 def getValue(self,key): 207 return getattr(self,key,None) 208 209 def getValueorDefault(self,key): 210 value = getattr(self,key,None) 211 if value is None: 212 field = self.__mappings__[key] 213 if field.default is not None: 214 value = field.default() if callable(field.default) else field.default 215 logging.debug('using default value for %s: %s' % (key,str(value))) 216 setattr(self,key,value) 217 218 return value 219 220 #ORM框架下，每条记录作为对象返回 221 #@classmethod定义类方法，类对象cls便可完成某些操作 222 @classmethod 223 async def findAll(cls,where=None,args=None,**kw): 224 sql = [cls.__select__] 225 #添加WHERE子句 226 if where: 227 sql.append('WHERE') 228 sql.append(where) 229 230 if args is None: 231 args = [] 232 233 orderby = kw.get('orderby',None) 234 #添加ORDER BY子句 235 if orderby: 236 sql.append('ORDER BY') 237 sql.append(orderby) 238 239 limit = kw.get('limit',None) 240 #添加LIMIT子句 241 if limit: 242 sql.append('LIMIT') 243 if isinstance(limit,int): 244 sql.append('?') 245 args.append(limit) 246 elif isinstance(limit,tuple): 247 sql.append('?, ?') 248 args.extend(limit) 249 else: 250 raise ValueError('Invalid limit value: %s' % str(limit)) 251 252 #execute SQL 253 rs = await select(' '.join(sql),args) 254 #将每条记录作为对象返回 255 return [cls(**r) for r in rs] 256 257 258 #过滤结果数量 259 @classmethod 260 async def findNumber(cls,selectField,where=None,args=None): 261 sql = ['SELECT %s _num_ from `%s`' % (selectField,cls.__table__)] 262 263 #添加WHERE子句 264 if where: 265 sql.append('WHERE') 266 sql.append(where) 267 268 rs = await select(' '.join(sql),args) 269 if len(rs) == 0: 270 return None 271 return rs[0]['_num_'] 272 273 #返回主键的一条记录 274 @classmethod 275 async def find(cls,pk): 276 rs = await select('%s WHERE `%s` = ?' % (cls.__select__,cls.__primary_key__),[pk],1) 277 if len(rs) == 0: 278 return None 279 return cls(**rs[0]) 280 281 #INSERT command 282 async def save(self): 283 args = list(map(self.getValueorDefault,self.__fields__)) 284 args.append(self.getValueorDefault(self.__primary_key__)) 285 rows = await execute(self.__insert__,args) 286 if rows != 1: 287 logging.warn('Faield to insert record:affected rows: %s' % rows) 288 289 #UPDATE command 290 async def update(self): 291 args = list(map(self.getValue,self.__fields__)) 292 args.append(self.getValue(self.__primary_key__)) 293 rows = await execute(self.__update__,args) 294 if rows != 1: 295 logging.warn('Faield to update by primary_key:affectesd rows: %s' % rows) 296 297 #DELETE command 298 async def remove(self): 299 args = [self.getValue(self.__primary_key__)] 300 rows = await execute(self.__delete__,args) 301 if rows != 1: 302 logging.warn('Faield to remove by primary key:affected: %s' % rows)