什么是ORM?
ORM全称“Object Relational Mapping”，即对象-关系映射，就是把关系数据库的一行映射为一个对象，也就是一个类对应一个表，这样，写代码更简单，不用直接操作SQL语句。
现在我们就要实现简易版ORM。
class Person(Model):
"""
定义类的属性到列的映射
"""
pid = IntegerField('id')
names = StringField('username')
email = StringField('email')
password = StringField('password')
p = Person(pid=10086, names='晓明', email='10086@163.com', password='123456')
p.save()
通过执行save()方法 动态生成sql插入语句, 是不是很神奇， 那我们现在开始解析原理吧
首先我们要定义一个 Field 类 它负责保存数据库表的字段名和字段类型：
class Field(object):
def __init__(self, name, column_type):
self.name = name
self.column_type = column_type
def __str__(self):
return '<%s:%s>' % (self.__class__.__name__, self.name)
在 Field 的基础上，进一步定义各种类型的 Field，比如 StringField，IntegerField 等等：
class StringField(Field):
def __init__(self, name):
super(StringField, self).__init__(name, 'varchar(100)')
class IntegerField(Field):
def __init__(self, name):
super(IntegerField, self).__init__(name, 'bigint')
下一步，就是编写最复杂的 ModelMetaclass：
class ModelMetaclass(type):
def __new__(cls, name, bases, attrs):
if name == "Model":
return type.__new__(cls, name, bases, attrs)
mappings = dict()
print("Found class: %s" % name)
for k, v in attrs.items():
if isinstance(v, Field):
print("Found mapping: %s ==> %s" % (k, v))
mappings[k] = v
for k in mappings.keys():
attrs.pop(k)
attrs["__table__"] = name # 表名和类名一致
attrs["__mappings__"] = mappings # 保存属性和列的映射关系
return type.__new__(cls, name, bases, attrs)最后就是基类 Model:
class Model(metaclass=ModelMetaclass):
def __init__(self, **kwargs):
_setattr = setattr
if kwargs:
for k, v in kwargs.items():
_setattr(self, k, v)
super(Model, self).__init__()
def save(self):
fields = []
params = []
args = []
for k, v in self.__mappings__.items():
fields.append(k)
params.append("?")
args.append(getattr(self, k, None))
sql = "insert into %s (%s) values (%s)" % (self.__table__, ','.join(fields), ",".join(params))
print('插入语句: %s' % sql)
print('参数: %s' % str(args))
def update(self):
fields = []
args = []
for k, v in self.__mappings__.items():
if getattr(self, k, None):
fields.append(k+"=?")
args.append(getattr(self, k, None))
sql = "update %s set %s" % (self.__table__, ','.join(fields))
print("更新语句: %s " % sql)
print("参数: %s" % args)
def filter(self, *args):
pass
def delete(self):
pass当用户定义一个 class Person(Model) 继承父类时，Python解释器会在当前类 Person 的定义中找 __metaclass__，如果没有找到，就继续到父类中找 __metaclass__，实在找不到就用默认 type 类。
我们在父类 Model 中定义了 __metaclass__ 的 ModelMetaclass 来创建 Person 类，所以 metaclass 隐式地继承到子类。
在 ModelMetaclass 中，一共做了几件事情：
在Model类中，就可以定义各种操作数据库的方法，比如save()，delete()，find()，update() 等等。
我们实现了save(), update()方法，把一个实例保存到数据库中。因为有表名，属性到字段的映射和属性值的集合，就可以构造出INSERT语句和UPDATE语句。
编写代码试试：
class UserInfo(Model):
"""
定义类的属性到列的映射
"""
uid = IntegerField('uid')
name = StringField('username')
email = StringField('email')
password = StringField('password')
class Person(Model):
"""
定义类的属性到列的映射
"""
pid = IntegerField('id')
names = StringField('username')
email = StringField('email')
password = StringField('password')
p = Person(pid=10086, names='晓明', email='10086@163.com', password='123456')
p.save()
u2 = UserInfo(password='123456')
u2.update()输出
Found class: UserInfo
Found mapping: uid ==> <IntegerField:uid>
Found mapping: name ==> <StringField:username>
Found mapping: email ==> <StringField:email>
Found mapping: password ==> <StringField:password>
Found class: Person
Found mapping: pid ==> <IntegerField:id>
Found mapping: names ==> <StringField:username>
Found mapping: email ==> <StringField:email>
Found mapping: password ==> <StringField:password>
插入语句: insert into Person (pid,names,email,password) values (?,?,?,?)
参数: [10086, '晓明', '10086@163.com', '123456']
更新语句: update UserInfo set password=?
参数: ['123456']
就这样一个小巧的ORM就这么完成了。是不是学到了很多呢 ？这里利用的是元编程，很多Python框架都运用了元编程达到动态操作类。
注：上述代码列子 结合了廖雪峰的列子和少量的django ORM源码。
class Field(object):
def __init__(self, name, column_type):
self.name = name
self.column_type = column_type
def __str__(self):
return '<%s:%s>' % (self.__class__.__name__, self.name)
class StringField(Field):
def __init__(self, name):
super(StringField, self).__init__(name, 'varchar(100)')
class IntegerField(Field):
def __init__(self, name):
super(IntegerField, self).__init__(name, 'bigint')
class ModelMetaclass(type):
def __new__(cls, name, bases, attrs):
if name == "Model":
return type.__new__(cls, name, bases, attrs)
mappings = dict()
print("Found class: %s" % name)
for k, v in attrs.items():
if isinstance(v, Field):
print("Found mapping: %s ==> %s" % (k, v))
mappings[k] = v
for k in mappings.keys():
attrs.pop(k)
attrs["__table__"] = name # 表名和类名一致
attrs["__mappings__"] = mappings # 保存属性和列的映射关系
return type.__new__(cls, name, bases, attrs)
class Model(metaclass=ModelMetaclass):
def __init__(self, **kwargs):
_setattr = setattr
if kwargs:
for k, v in kwargs.items():
_setattr(self, k, v)
super(Model, self).__init__()
def save(self):
fields = []
params = []
args = []
for k, v in self.__mappings__.items():
fields.append(k)
params.append("?")
args.append(getattr(self, k, None))
sql = "insert into %s (%s) values (%s)" % (self.__table__, ','.join(fields), ",".join(params))
print('插入语句: %s' % sql)
print('参数: %s' % str(args))
def update(self):
fields = []
args = []
for k, v in self.__mappings__.items():
if getattr(self, k, None):
fields.append(k+"=?")
args.append(getattr(self, k, None))
sql = "update %s set %s" % (self.__table__, ','.join(fields))
print("更新语句: %s " % sql)
print("参数: %s" % args)
def filter(self, *args):
pass
def delete(self):
pass
class UserInfo(Model):
"""
定义类的属性到列的映射
"""
uid = IntegerField('uid')
name = StringField('username')
email = StringField('email')
password = StringField('password')
class Person(Model):
"""
定义类的属性到列的映射
"""
pid = IntegerField('id')
names = StringField('username')
email = StringField('email')
password = StringField('password')
p = Person(pid=10086, names='晓明', email='10086@163.com', password='123456')
p.save()
u2 = UserInfo(password='123456')
u2.update()
