通过metaclass实现精简的ORM框架

摘抄于《python-3-廖雪峰》
使用ORM框架

class User(Model):
    #定义类的属性到列的映射：
    id = IntegerFiled('id')
    name = StringField('username')
    email = StringField('email')
    password = StringField('password')

#创建一个实例
u = User(id = 12345, name='Michael', email='test@orm.org',password='my-pwd')
#保存到数据库
u.save()

使用上面的接口来实现ORM
使用field类，它负责数据库表的字段名和字段类型：

class Field(object):
    
    def __init__(self, name, column_type):
        self.name = name
        self.column_type = column_type
        
    def __str__(self):
        return '<%s:%s>' % (self.__class__.__name__, self.name)

定义StringField，IntegerField

class StringField(Field):
    
    def __init__(self, name):
        super(StringField, self).__init__(name, 'varchar(100)')
        
class IntegerField(Field):
    
    def __init__(self, name):
        super(IntegerField, self).__init__(name, 'bigint')

编写ModelMetaclass

class ModelMetaclass(type):
    
    def __new__(cls, name, bases, attrs):
        if name=='Model':
            return type.__new__(cls, name, bases, attrs)
        print('Found model: %s' % name)
        mappings = dict()
        for k, v in attrs.items():
            if isinstance(v, Field):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v
        for k in mappings.keys():
            atrrs.pop(k)
        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
        reutrn type.__new__(cls, name, bases, attrs)

基类Model：

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 save(self_:
        fileds = []
        params = []
        args = []
        for k, v in self.__mappings__.item():
            fields.append(v.name)
            params.append('?')
            args.append(getattr(self, k, None))
        sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fileds), ','.join(params))
        print('SQL: %s' % sql)
        print('ARGS: %s %  str(args))