ORM重要Django的核心思想

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





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， 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

通过元类简单实现ORM中的insert功能



执行的效果:



完善对数据类型的检测











class ModelMetaclass(type):

def new(cls, name, bases, attrs):

mappings = dict()

# 判断是否需要保存

for k, v in attrs.items():

# 判断是否是指定的StringField或者IntegerField的实例对象

if isinstance(v, tuple):

print(‘Found mapping: %s ==> %s’ % (k, v))

mappings[k] = v

# 删除这些已经在字典中存储的属性
for k in mappings.keys():
attrs.pop(k)

# 将之前的uid/name/email/password以及对应的对象引用、类名字
attrs['__mappings__'] = mappings  # 保存属性和列的映射关系
attrs['__table__'] = name  # 假设表名和类名一致
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)”)

# 当指定元类之后，以上的类属性将不在类中，而是在mappings属性指定的字典中存储

# 以上User类中有

# 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:
# 判断入如果是数字类型
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()

运行效果如下:

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. 抽取到基类中

class ModelMetaclass(type):

def new(cls, name, bases, attrs):

mappings = dict()

# 判断是否需要保存

for k, v in attrs.items():

# 判断是否是指定的StringField或者IntegerField的实例对象

if isinstance(v, tuple):

print(‘Found mapping: %s ==> %s’ % (k, v))

mappings[k] = v

# 删除这些已经在字典中存储的属性
for k in mappings.keys():
attrs.pop(k)

# 将之前的uid/name/email/password以及对应的对象引用、类名字
attrs['__mappings__'] = mappings  # 保存属性和列的映射关系
attrs['__table__'] = name  # 假设表名和类名一致
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:
# 判断入如果是数字类型
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()