8.6 通过元素实现ORM

首先明确需求

# 简单定义
class User:
    name = CharFiled(db_column="", max_length=32)
    age = IntFiled(db_column="", min_value=0, max_value=100)
    class Meta:
        db_table = 'user'

# ORM
user = User()
user.name = 'linda'
user.age = 18
user.save()

迷你版 ORM

from collections import OrderedDict


class Field:
    pass


class IntField(Field):
    def __init__(self, db_column, min_value=0, max_value=100):
        self.db_column = db_column
        self.min_value = min_value
        self.max_value = max_value
        self._value = None

    def __get__(self, instance, owner):
        return self._value

    def __set__(self, instance, value):
        if not isinstance(value, int):
            raise TypeError('need int value')
        if value < self.min_value or value > self.max_value:
            raise ValueError('need [%s, %s] value' % (self.min_value, self.max_value))
        self._value = value


class CharField(Field):
    def __init__(self, db_column, max_length=32):
        self.db_column = db_column
        self.max_length = max_length
        self._value = None

    def __get__(self, instance, owner):
        return self._value

    def __set__(self, instance, value):
        if not isinstance(value, str):
            raise TypeError('need str value')
        if len(value) > self.max_length:
            raise ValueError('len need lower than %s' % self.max_length)
        self._value = value


# 元类注入一系列属性
class MetaClass(type):
    def __new__(cls, name, bases, attrs, **kw):
        # BaseModel 也会调用 Metaclass，但没有定义 name，age 等属性，可特殊判断
        if name == 'BaseModel':
            return super().__new__(cls, name, bases, attrs, **kw)

        fields = {}
        for key, value in attrs.items():
            if isinstance(value, Field):
                fields[key] = value

        attrs_meta = attrs.get('Meta', None)
        _meta = {}
        db_table = name.lower()
        if attrs_meta is not None:
            table = getattr(attrs_meta, 'db_table', None)
            if not table:
                db_table = table

        _meta['db_table'] = db_table
        attrs['_meta'] = _meta
        attrs['fields'] = fields
        if attrs.get('Meta'):
            del attrs['Meta']
        return super().__new__(cls, name, bases, attrs, **kw)


class BaseModel(metaclass=MetaClass):
    def __init__(self, **kw):
        for key, value in kw.items():
            setattr(self, key, value)
        super().__init__()

    def save(self):
        fields = OrderedDict(self.fields)
        fields_str = ", ".join([value.db_column for value in fields.values()])
        values_str = ', '.join([str(getattr(self, field)) if not isinstance(value, CharField)
                                else "'%s'" % str(getattr(self, field))
                                for field, value in fields.items()])
        sql = "insert into %s (%s) values (%s)" % (self._meta['db_table'], fields_str, values_str)
        print(sql)
        # insert into user (name1, age) values ('linda', 20)


# 自定义类时写少量属性，元类帮助我们注入很多通用属性
class User(BaseModel):
    name = CharField('name1', max_length=16)
    age = IntField('age', min_value=0, max_value=100)

    class Meta:
        db_table = 'user'


if __name__ == '__main__':
    user = User(name='linda')
    user.age = 20
    user.save()

ORM 设计思想

• 数据属性描述符(__set__, __get__) 实现验证操作
• 自定义元类(MetaClass(type)) 实现参数注入
• 自定义 ORM 类(BaseModel) 获取元类注入的参数 进行额外操作
• 自定义元类 注入 objects
• 需特别注意调用层级顺序，__new__ 在 __init__ 之前，所以 __init__ 中可以使用元类注册测参数