元類實現ORM
1. ORM是什么
ORM 是 python編程語言后端web框架 Django的核心思想，“Object Relational Mapping”，即對象-關系映射，簡稱ORM。
一個句話理解就是：創建一個實例對象，用創建它的類名當做數據表名，用創建它的類屬性對應數據表的字段，當對這個實例對象操作時，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='test@orm.org', password='my-pwd')
u.save()
# 對應如下sql語句
# insert into User (username,email,password,uid)
# values ('Michael','test@orm.org','my-pwd',12345)
說明
- 所謂的ORM就是讓開發者在操作數據庫的時候，能夠像操作對象時通過
xxxx.屬性=yyyy一樣簡單，這是開發ORM的初衷 - 只不過ORM的實現較為復雜，Django中已經實現了 很復雜的操作，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()# 判斷是否需要保存for k, v in attrs.items():# 判斷是否是指定的StringField或者IntegerField的實例對象if isinstance(v, tuple):print('Found mapping: %s ==> %s' % (k, v))mappings[k] = v# 刪除這些已經在字典中存儲的屬性for k in mappings.keys():attrs.pop(k)# 將之前的uid/name/email/password以及對應的對象引用、類名字attrs['__mappings__'] = mappings # 保存屬性和列的映射關系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)")# 當指定元類之后，以上的類屬性將不在類中，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))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='test@orm.org', password='my-pwd')
# print(u.__dict__)
u.save()
執行的效果:（sql語句中缺少單引號）
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,test@orm.org)
3. 完善對數據類型的檢測
class ModelMetaclass(type):def __new__(cls, name, bases, attrs):mappings = dict()# 判斷是否需要保存for k, v in attrs.items():# 判斷是否是指定的StringField或者IntegerField的實例對象if isinstance(v, tuple):print('Found mapping: %s ==> %s' % (k, v))mappings[k] = v# 刪除這些已經在字典中存儲的屬性for k in mappings.keys():attrs.pop(k)# 將之前的uid/name/email/password以及對應的對象引用、類名字attrs['__mappings__'] = mappings # 保存屬性和列的映射關系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)")# 當指定元類之后，以上的類屬性將不在類中，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='test@orm.org', password='my-pwd')
# print(u.__dict__)
u.save()
運行效果如下:
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 ('test@orm.org',12345,'my-pwd','Michael')
4. 抽取到基類中
class ModelMetaclass(type):def __new__(cls, name, bases, attrs):mappings = dict()# 判斷是否需要保存for k, v in attrs.items():# 判斷是否是指定的StringField或者IntegerField的實例對象if isinstance(v, tuple):print('Found mapping: %s ==> %s' % (k, v))mappings[k] = v# 刪除這些已經在字典中存儲的屬性for k in mappings.keys():attrs.pop(k)# 將之前的uid/name/email/password以及對應的對象引用、類名字attrs['__mappings__'] = mappings # 保存屬性和列的映射關系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)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='test@orm.org', password='my-pwd')
# print(u.__dict__)
u.save()
