ORM实现原理

class ModelMetaclass(type):
def new(cls, name, bases, attrs):
# name=”User”
# bases=(object,)
# attrs = {
# “uid”: (‘uid’, “int unsigned”),
# “name”: (‘username’, “varchar(30)”),
# “email”: (‘email’, “varchar(30)”),
# “password”: (‘password’, “varchar(30)”)
# “save”: xxxxxx,
# “init“: xxxxxx2
# }
mappings = dict()
# mappings = {
# “uid”: (‘uid’, “int unsigned”),
# “name”: (‘username’, “varchar(30)”),
# “email”: (‘email’, “varchar(30)”),
# “password”: (‘password’, “varchar(30)”)
# }
# 判断是否需要保存
for k, v in attrs.items():
# 判断是否是元组类型
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)  # del attrs[k]

    # attrs = {
    #     "save": xxxxxx,
    #     "__init__": xxxxxx2，
    #     "__mappings__": {
    #           "uid": ('uid', "int unsigned"),
    #           "name": ('username', "varchar(30)"),
    #           "email": ('email', "varchar(30)"),
    #           "password": ('password', "varchar(30)")
    #     },
    #     "__table__": "User"
    # }

    # 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=’[email protected]’, password=’my-pwd’)

print(u.dict)

u.save()