使用python 元類實現ORM關係對映
阿新 • • 發佈:2018-12-03
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
- 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 - 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()