1. ORM是什么
ORM 是 python编程语言后端web框架 Django的核心思想,“Object Relational Mapping”,即对象-关系映射,简称ORM。
一个句话理解就是:创建一个实例对象,用创建它的类名当做数据表名,用创建它的类属性对应数据表的字段,当对这个实例对象操作时,能够对应MySQL语句
demo:
class User(父类省略):
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()
# 对应如下sql语句
# insert into User (username,email,password,uid)
# values ('Michael','[email protected]','my-pwd',12345)
说明
- 所谓的ORM就是让开发者在操作数据库的时候,能够像操作对象时通过
xxxx.属性=yyyy
一样简单,这是开发ORM的初衷 - 只不过ORM的实现较为复杂,Django中已经实现了 很复杂的操作,本节知识 主要通过完成一个 insert相类似的ORM,理解其中的道理就就可以了
2. 通过元类简单实现ORM中的insert功能
class ModelMetaClass(type):
def __new__(cls, class_name, class_parents, class_attrs):
mapping = dict()
for name, value in class_attrs.items():
if isinstance(value, tuple):
print("Found mapping %s --> %s" % (name, value))
mapping[name] = value
for name in mapping.keys():
class_attrs.pop(name)
class_attrs["__mapping__"] = mapping
class_attrs["__table__"] = class_name
return type.__new__(cls, class_name, class_parents, class_attrs)
class User(metaclass=ModelMetaClass):
uid = ('uid', "int unsigned")
name = ('username', "varchar(30)")
email = ('email', "varchar(30)")
password = ('password', "varchar(30)")
def __init__(self, **kwargs):
for name, value in kwargs.items():
setattr(self, name, value)
def save(self):
fields = []
args = []
for k, v in self.__mapping__.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=123, name="zfx", email="[email protected]", password="6666")
u.save()
执行的效果:
Found mapping uid --> ('uid', 'int unsigned')
Found mapping name --> ('username', 'varchar(30)')
Found mapping password --> ('password', 'varchar(30)')
Found mapping email --> ('email', 'varchar(30)')
SQL: insert into User(uid,password,email,username) values(123,6666,[email protected],zfx)
3. 完善对数据类型的检测
class ModelMetaClass(type):
def __new__(cls, class_name, class_parents, class_attrs):
mapping = dict()
for name, value in class_attrs.items():
if isinstance(value, tuple):
print("Found mapping %s --> %s" % (name, value))
mapping[name] = value
for name in mapping.keys():
class_attrs.pop(name)
class_attrs["__mapping__"] = mapping
class_attrs["__table__"] = class_name
return type.__new__(cls, class_name, class_parents, class_attrs)
class User(metaclass=ModelMetaClass):
uid = ('uid', "int unsigned")
name = ('username', "varchar(30)")
email = ('email', "varchar(30)")
password = ('password', "varchar(30)")
def __init__(self, **kwargs):
for name, value in kwargs.items():
setattr(self, name, value)
def save(self):
fields = []
args = []
for k, v in self.__mapping__.items():
fields.append(v[0])
args.append(getattr(self, k, None))
args_v = []
for arg in args:
if isinstance(arg, int):
args_v.append(str(arg))
else:
args_v.append("""'%s'""" % arg)
sql = "insert into %s(%s) values(%s)" % (self.__table__, ",".join(fields), ",".join(args_v))
print("SQL: %s" % sql)
u = User(uid=123, name="zfx", email="[email protected]", password="6666")
u.save()
执行的效果:
Found mapping uid --> ('uid', 'int unsigned')
Found mapping password --> ('password', 'varchar(30)')
Found mapping email --> ('email', 'varchar(30)')
Found mapping name --> ('username', 'varchar(30)')
SQL: insert into User(password,uid,username,email) values('6666',123,'zfx','[email protected]')
4. 抽取到基类中
class ModelMetaClass(type):
def __new__(cls, class_name, class_parents, class_attrs):
mapping = dict()
for name, value in class_attrs.items():
if isinstance(value, tuple):
print("Found mapping %s --> %s" % (name, value))
mapping[name] = value
for name in mapping.keys():
class_attrs.pop(name)
class_attrs["__mapping__"] = mapping
class_attrs["__table__"] = class_name
return type.__new__(cls, class_name, class_parents, class_attrs)
class Model(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.__mapping__.items():
fields.append(v[0])
args.append(getattr(self, k, None))
args_v = []
for arg in args:
if isinstance(arg, int):
args_v.append(str(arg))
else:
args_v.append("""'%s'""" % arg)
sql = "insert into %s(%s) values(%s)" % (self.__table__, ",".join(fields), ",".join(args_v))
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=123, name="zfx", email="[email protected]", password="6666")
u.save()
原文地址:https://www.cnblogs.com/zhangfengxian/p/10223817.html
时间: 2024-10-08 20:15:24