元類實現ORM

1. ORM是什麼

ORM 是 python程式語言後端web框架 Django的核心思想，“Object Relational Mapping”，即物件-關係對映，簡稱ORM。

一個句話理解就是：建立一個例項物件，用建立它的類名當做資料表名，用建立它的類屬性對應資料表的欄位，當對這個例項物件操作時，能夠對應MySQL語句

demo:

class User(父類省略):
    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='
 
[email protected]', password='my-pwd')
u.save()
# 對應如下sql語句
# insert into User (username,email,password,uid)
# values ('Michael','[email protected]','my-pwd',12345)

說明

1. 所謂的ORM就是讓開發者在操作資料庫的時候，能夠像操作物件時通過xxxx.屬性=yyyy一樣簡單，這是開發ORM的初衷
2. 只不過ORM的實現較為複雜，Django中已經實現了 很複雜的操作，本節知識 主要通過完成一個 insert相類似的ORM，理解其中的道理就就可以了

2. 通過元類簡單實現ORM中的insert功能

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的例項物件
            if isinstance(v, tuple):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v

        # 刪除這些已經在字典中儲存的屬性
        for k in mappings.keys():
            attrs.pop(k)

        # 將之前的uid/name/email/password以及對應的物件引用、類名字
        attrs['__mappings__'] = mappings  # 儲存屬性和列的對映關係
        attrs['__table__'] = name  # 假設表名和類名一致
        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)")
    # 當指定元類之後，以上的類屬性將不在類中，而是在__mappings__屬性指定的字典中儲存
    # 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))

        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()

執行的效果:

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,[email protected])

3. 完善對資料型別的檢測

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的例項物件
            if isinstance(v, tuple):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v

        # 刪除這些已經在字典中儲存的屬性
        for k in mappings.keys():
            attrs.pop(k)

        # 將之前的uid/name/email/password以及對應的物件引用、類名字
        attrs['__mappings__'] = mappings  # 儲存屬性和列的對映關係
        attrs['__table__'] = name  # 假設表名和類名一致
        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)")
    # 當指定元類之後，以上的類屬性將不在類中，而是在__mappings__屬性指定的字典中儲存
    # 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:
            # 判斷入如果是數字型別
            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()

執行效果如下:

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 ('[email protected]',12345,'my-pwd','Michael')

4. 抽取到基類中

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的例項物件
            if isinstance(v, tuple):
                print('Found mapping: %s ==> %s' % (k, v))
                mappings[k] = v

        # 刪除這些已經在字典中儲存的屬性
        for k in mappings.keys():
            attrs.pop(k)

        # 將之前的uid/name/email/password以及對應的物件引用、類名字
        attrs['__mappings__'] = mappings  # 儲存屬性和列的對映關係
        attrs['__table__'] = name  # 假設表名和類名一致
        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:
            # 判斷入如果是數字型別
            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='[email protected]', password='my-pwd')
# print(u.__dict__)
u.save()