Python高级-元类实现ORM-笔记

1. ORM是什么

ORM 是 python编程语言后端web框架 Django的核心思想，“Object Relational Mapping”，即对象-关系映射，简称ORM。

一个句话理解就是：创建一个实例对象，用创建它的类名当做数据表名，用创建它的类属性对应数据表的字段，当对这个实例对象操作时，能够对应MySQL语句

demo:

class User(父类省略): uid = ('uid', "int unsigned") name = ('username', "varchar(30)") email = ('email', "varchar(30)") password = ('password', "varchar(30)") ...省略...u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd')u.save()# 对应如下sql语句# insert into User (username,email,password,uid)# values ('Michael','test@orm.org','my-pwd',12345)

说明

所谓的ORM就是让开发者在操作数据库的时候，能够像操作对象时通过​​xxxx.属性=yyyy​​一样简单，这是开发ORM的初衷

只不过ORM的实现较为复杂，Django中已经实现了 很复杂的操作，本节知识 主要通过完成一个 insert相类似的ORM，理解其中的道理就就可以了

2. 通过元类简单实现ORM中的insert功能

class ModelMetaclass(type): def __new__(cls, name, bases, attrs): mappings = dict() # 判断是否需要保存 for k, v in attrs.items(): # 判断是否是指定的StringField或者IntegerField的实例对象 if isinstance(v, tuple): print('Found mapping: %s ==> %s' % (k, v)) mappings[k] = v # 删除这些已经在字典中存储的属性 for k in mappings.keys(): attrs.pop(k) # 将之前的uid/name/email/password以及对应的对象引用、类名字 attrs['__mappings__'] = mappings # 保存属性和列的映射关系 attrs['__table__'] = name # 假设表名和类名一致 return type.__new__(cls, name, bases, attrs)class User(metaclass=ModelMetaclass): uid = ('uid', "int unsigned") name = ('username', "varchar(30)") email = ('email', "varchar(30)") password = ('password', "varchar(30)") # 当指定元类之后，以上的类属性将不在类中，而是在__mappings__属性指定的字典中存储 # 以上User类中有 # __mappings__ = { # "uid": ('uid', "int unsigned") # "name": ('username', "varchar(30)") # "email": ('email', "varchar(30)") # "password": ('password', "varchar(30)") # } # __table__ = "User" def __init__(self, **kwargs): for name, value in kwargs.items(): setattr(self, name, value) def save(self): fields = [] args = [] for k, v in self.__mappings__.items(): fields.append(v[0]) args.append(getattr(self, k, None)) sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join([str(i) for i in args])) print('SQL: %s' % sql)u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd')# print(u.__dict__)u.save()

执行的效果:

Found mapping: password ==> ('password', 'varchar(30)')Found mapping: email ==> ('email', 'varchar(30)')Found mapping: uid ==> ('uid', 'int unsigned')Found mapping: name ==> ('username', 'varchar(30)')SQL: insert into User (uid,password,username,email) values (12345,my-pwd,Michael,test@orm.org)

3. 完善对数据类型的检测

class ModelMetaclass(type): def __new__(cls, name, bases, attrs): mappings = dict() # 判断是否需要保存 for k, v in attrs.items(): # 判断是否是指定的StringField或者IntegerField的实例对象 if isinstance(v, tuple): print('Found mapping: %s ==> %s' % (k, v)) mappings[k] = v # 删除这些已经在字典中存储的属性 for k in mappings.keys(): attrs.pop(k) # 将之前的uid/name/email/password以及对应的对象引用、类名字 attrs['__mappings__'] = mappings # 保存属性和列的映射关系 attrs['__table__'] = name # 假设表名和类名一致 return type.__new__(cls, name, bases, attrs)class User(metaclass=ModelMetaclass): uid = ('uid', "int unsigned") name = ('username', "varchar(30)") email = ('email', "varchar(30)") password = ('password', "varchar(30)") # 当指定元类之后，以上的类属性将不在类中，而是在__mappings__属性指定的字典中存储 # 以上User类中有 # __mappings__ = { # "uid": ('uid', "int unsigned") # "name": ('username', "varchar(30)") # "email": ('email', "varchar(30)") # "password": ('password', "varchar(30)") # } # __table__ = "User" def __init__(self, **kwargs): for name, value in kwargs.items(): setattr(self, name, value) def save(self): fields = [] args = [] for k, v in self.__mappings__.items(): fields.append(v[0]) args.append(getattr(self, k, None)) args_temp = list() for temp in args: # 判断入如果是数字类型 if isinstance(temp, int): args_temp.append(str(temp)) elif isinstance(temp, str): args_temp.append("""'%s'""" % temp) sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(args_temp)) print('SQL: %s' % sql)u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd')# print(u.__dict__)u.save()

运行效果如下:

Found mapping: uid ==> ('uid', 'int unsigned')Found mapping: password ==> ('password', 'varchar(30)')Found mapping: name ==> ('username', 'varchar(30)')Found mapping: email ==> ('email', 'varchar(30)')SQL: insert into User (email,uid,password,username) values ('test@orm.org',12345,'my-pwd','Michael')

4. 抽取到基类中

class ModelMetaclass(type): def __new__(cls, name, bases, attrs): mappings = dict() # 判断是否需要保存 for k, v in attrs.items(): # 判断是否是指定的StringField或者IntegerField的实例对象 if isinstance(v, tuple): print('Found mapping: %s ==> %s' % (k, v)) mappings[k] = v # 删除这些已经在字典中存储的属性 for k in mappings.keys(): attrs.pop(k) # 将之前的uid/name/email/password以及对应的对象引用、类名字 attrs['__mappings__'] = mappings # 保存属性和列的映射关系 attrs['__table__'] = name # 假设表名和类名一致 return type.__new__(cls, name, bases, attrs)class Model(object, metaclass=ModelMetaclass): def __init__(self, **kwargs): for name, value in kwargs.items(): setattr(self, name, value) def save(self): fields = [] args = [] for k, v in self.__mappings__.items(): fields.append(v[0]) args.append(getattr(self, k, None)) args_temp = list() for temp in args: # 判断入如果是数字类型 if isinstance(temp, int): args_temp.append(str(temp)) elif isinstance(temp, str): args_temp.append("""'%s'""" % temp) sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(args_temp)) print('SQL: %s' % sql)class User(Model): uid = ('uid', "int unsigned") name = ('username', "varchar(30)") email = ('email', "varchar(30)") password = ('password', "varchar(30)")u = User(uid=12345, name='Michael', email='test@orm.org', password='my-pwd')# print(u.__dict__)u.save()