Python--如何利用元類實作ORM

元類實作ORM

ORM是Django的核心思想，“Object Relational Mapping”，即對象-關系映射，用于簡化SQL操作，把資料庫的操作封裝到類裡面，把表名映射成類，把字段映射為屬性，把行（資料）映射為執行個體。

class ModelMetaclass(type):
    def __new__(cls, name, bases, attrs):
        mappings = dict()
        # 判斷是否需要儲存
        for k, v in attrs.items():
            # 判斷是否是指定的StringField或者IntegerField的執行個體對象
            if isinstance(v, tuple):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v

        # 删除這些已經在字典中存儲的屬性
        for k in mappings.keys():
            attrs.pop(k)

        # 将之前的uid/name/email/password以及對應的對象引用、類名字
        attrs['__mappings__'] = mappings  # 儲存屬性和列的映射關系
        attrs['__table__'] = name  # 假設表名和類名一緻
        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)")
    # 當指定元類之後，以上的類屬性将不在類中，而是在__mappings__屬性指定的字典中存儲
    # 以上User類中有 
    # __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:
            # 判斷入如果是數字類型
            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='[email protected]', password='my-pwd')
# print(u.__dict__)
u.save()
           

注：

• setattr(對象, 屬性, 屬性值) 給對象中的屬性設定值
• getattr(對象, 屬性[, 預設值]) 從對象中讀取屬性的值，可以設定預設值，如果不存在則讀取預設值。