元類實(shí)現(xiàn)ORM
ORM是什么
ORM 是 python編程語(yǔ)言后端web框架 Django的核心思想，“Object Relational Mapping”源武，
即對(duì)象-關(guān)系映射，簡(jiǎn)稱ORM。
一個(gè)句話理解就是：創(chuàng)建一個(gè)實(shí)例對(duì)象笛辟，用創(chuàng)建它的類名當(dāng)做數(shù)據(jù)表名藐不，用創(chuàng)建它的類屬性對(duì)應(yīng)數(shù)據(jù)表的字段旭咽，
當(dāng)對(duì)這個(gè)實(shí)例對(duì)象操作時(shí)，能夠?qū)?yīng)MySQL語(yǔ)句
說(shuō)明
所謂的ORM就是讓開發(fā)者在操作數(shù)據(jù)庫(kù)的時(shí)候窒篱，能夠像操作對(duì)象時(shí)通過(guò)xxxx.屬性=yyyy一樣簡(jiǎn)單，
這是開發(fā)ORM的初衷
只不過(guò)ORM的實(shí)現(xiàn)較為復(fù)雜，Django中已經(jīng)實(shí)現(xiàn)了很復(fù)雜的操作墙杯，我們主要理解其中的意義
通過(guò)元類簡(jiǎn)單實(shí)現(xiàn)ORM中的insert功能
定義元類
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(shí)例對(duì)象
if isinstance(v, tuple):
print('Found mapping: %s ==> %s' % (k, v))
mappings[k] = v
# 刪除這些已經(jīng)在字典中存儲(chǔ)的屬性
for k in mappings.keys():
attrs.pop(k)
# 將之前的uid/name/email/password以及對(duì)應(yīng)的對(duì)象引用配并、類名字
attrs['__mappings__'] = mappings # 保存屬性和列的映射關(guān)系
attrs['__table__'] = name # 假設(shè)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(dāng)指定元類之后，以上的類屬性將不在類中高镐，而是在__mappings__屬性指定的字典中存儲(chǔ)
# 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("-"*70)
print(u.__dict__)
u.save()
打印結(jié)果：
Found mapping: uid ==> ('uid', 'int unsigned')
Found mapping: name ==> ('username', 'varchar(30)')
Found mapping: email ==> ('email', 'varchar(30)')
Found mapping: password ==> ('password', 'varchar(30)')
----------------------------------------------------------------------
{'uid': 12345, 'name': 'Michael', 'email': 'test@orm.org', 'password': 'my-pwd'}
SQL: insert into User (uid,username,email,password) values (12345,Michael,test@orm.org,my-pwd)
完善對(duì)數(shù)據(jù)類型的檢測(cè)
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(shí)例對(duì)象
if isinstance(v, tuple):
print('Found mapping: %s ==> %s' % (k, v))
mappings[k] = v
# 刪除這些已經(jīng)在字典中存儲(chǔ)的屬性
for k in mappings.keys():
attrs.pop(k)
# 將之前的uid/name/email/password以及對(duì)應(yīng)的對(duì)象引用溉旋、類名字
attrs['__mappings__'] = mappings # 保存屬性和列的映射關(guān)系
attrs['__table__'] = name # 假設(shè)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(dāng)指定元類之后，以上的類屬性將不在類中嫉髓，而是在__mappings__屬性指定的字典中存儲(chǔ)
# 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(shù)字類型
if isinstance(temp, int):
args_temp.append(str(temp))
elif isinstance(temp, str):
args_temp.append("""'%s'""" % temp)
# print(args_temp) # ['12345', "'Michael'", "'test@orm.org'", "'my-pwd'"]
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("-"*70)
print(u.__dict__)
u.save()
打印結(jié)果：
Found mapping: uid ==> ('uid', 'int unsigned')
Found mapping: name ==> ('username', 'varchar(30)')
Found mapping: email ==> ('email', 'varchar(30)')
Found mapping: password ==> ('password', 'varchar(30)')
----------------------------------------------------------------------
{'uid': 12345, 'name': 'Michael', 'email': 'test@orm.org', 'password': 'my-pwd'}
['12345', "'Michael'", "'test@orm.org'", "'my-pwd'"]
SQL: insert into User (uid,username,email,password) values (12345,'Michael','test@orm.org','my-pwd')
