浅谈python web框架中的orm设计

webtet

　　看了一下廖雪峰的那个web框架，其实就是封装了web.py,请求使用异步并将aiomysql做为MySQL数据库提供了异步IO的驱动，前端部分则整合了jinja.其中最难的应该是orm部分了。
　　下面是orm一个简单的例子。



class User(Model):
__table__ = 'users'
id = StringField(primary_key=True, default=next_id, ddl='varchar(50)')
email = StringField(ddl='varchar(50)')
passwd = StringField(ddl='varchar(50)')
admin = BooleanField()
name = StringField(ddl='varchar(50)')
image = StringField(ddl='varchar(500)')
created_at = FloatField(default=time.time)
　　整体来看这是一个为User的简单table，集成了Model类。然后字段则是各种类型Field。
　　1.先来看看字段,也就是Field部分。Field部分比较简单，继承了object,然后初始化一些字段，其实没什么好说的。在字段赋值时，参数以dict格式传入，传入到哪里呢，本来type的dict是为none的，但下面这段代码用__new__改变了原来的type，而元类是创建类的类，因此，Field字段的参数，都会在ModelMetaclass类中做处理，下面看个调试的例子，看看元类的威力。



IPython 4.0.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.
In [1]: from orm import Model
In [2]: class Blog(Model):
...:         pass
...:
In [3]:
In [3]: a=Blog
In [4]: type(a)
Out[4]: orm.ModelMetaclass
In [5]: dir(a)
Out[5]:
['__class__',
'__contains__',
'__delattr__',
'__delitem__',
'__dict__',
'__dir__',
'__doc__',
'__eq__',
'__fields__',
'__format__',
'__ge__',
'__getattr__',
'__getattribute__',
'__getitem__',
'__gt__',
'__hash__',
'__init__',
'__iter__',
'__le__',
'__len__',
'__lt__',
'__mappings__',
'__module__',
'__ne__',
'__new__',
'__primary_key__',
'__reduce__',
'__reduce_ex__',
'__repr__',
'__setattr__',
'__setitem__',
'__sizeof__',
'__str__',
'__subclasshook__',
'__table__',
'__weakref__',
'clear',
'copy',
'find',
'findAll',
'findNumber',
'fromkeys',
'get',
'getValue',
'getValueOrDefault',
'items',
'keys',
'pop',
'popitem',
'remove',
'save',
'setdefault',
'update',
'values']
In [6]:
　　从上面可以看到,a是继承了元类修改后的类Blog的一个对象，因此继承了新增的属性跟方法。所以元类都能对所有的orm中的表做出约束和修改。
　　下面这段继承自type的ModelMetaclass就是元类的代码。



class ModelMetaclass(type):
def __new__(cls, name, bases, attrs):
if name=='Model':
return type.__new__(cls, name, bases, attrs)
tableName = attrs.get('__table__', None) or name
logging.info('found model: %s (table: %s)' % (name, tableName))
mappings = dict()
fields = []
primaryKey = None
for k, v in attrs.items():
if isinstance(v, Field):
logging.info('  found mapping: %s ==> %s' % (k, v))
mappings[k] = v
if v.primary_key:
# 找到主键:
if primaryKey:
raise StandardError('Duplicate primary key for field: %s' % k)
primaryKey = k
else:
fields.append(k)
if not primaryKey:
raise StandardError('Primary key not found.')
for k in mappings.keys():
attrs.pop(k)
escaped_fields = list(map(lambda f: '`%s`' % f, fields))
attrs['__mappings__'] = mappings # 保存属性和列的映射关系
attrs['__table__'] = tableName
attrs['__primary_key__'] = primaryKey # 主键属性名
attrs['__fields__'] = fields # 除主键外的属性名
attrs['__select__'] = 'select `%s`, %s from `%s`' % (primaryKey, ', '.join(escaped_fields), tableName)
attrs['__insert__'] = 'insert into `%s` (%s, `%s`) values (%s)' % (tableName, ', '.join(escaped_fields), primaryKey, create_args_string(len(escaped_fields) + 1))
attrs['__update__'] = 'update `%s` set %s where `%s`=?' % (tableName, ', '.join(map(lambda f: '`%s`=?' % (mappings.get(f).name or f), fields)), primaryKey)
attrs['__delete__'] = 'delete from `%s` where `%s`=?' % (tableName, primaryKey)
return type.__new__(cls, name, bases, attrs)
　　而Model类相对简单，主要是为了初始化了get/set以及几个getvalue的函数,这一块根据自己需要看看就好。



class Model(dict, metaclass=ModelMetaclass):
def __init__(self, **kw):
super(Model, self).__init__(**kw)
def __getattr__(self, key):
try:
return self[key]
except KeyError:
raise AttributeError(r"'Model' object has no attribute '%s'" % key)
def __setattr__(self, key, value):
self[key] = value
def getValue(self, key):
return getattr(self, key, None)
def getValueOrDefault(self, key):
value = getattr(self, key, None)
if value is None:
field = self.__mappings__[key]
if field.default is not None:
value = field.default() if callable(field.default) else field.default
logging.debug('using default value for %s: %s' % (key, str(value)))
setattr(self, key, value)
return value
@classmethod
@asyncio.coroutine
def findNumber(cls, selectField, where=None, args=None):
' find number by select and where. '
sql = ['select %s _num_ from `%s`' % (selectField, cls.__table__)]
if where:
sql.append('where')
sql.append(where)
rs = yield from select(' '.join(sql), args, 1)
if len(rs) == 0:
return None
return rs[0]['_num_']
....
　　@classmethod是为了将方法变成类方法,@asyncio.coroutine则是为了做异步处理。
　　那每一个orm下面的表是如何进行值传递的呢？下面看看另一个调试例子



In [2]: from orm import Model,StringField
In [3]: class Blog(Model):
...:         __table__='user'
...:         id=StringField(ddl='varchar(50)')
...:         age=StringField(name="ss")
...:
In [4]: a = Blog
In [5]: dir(a)
Out[5]:
['__class__',
'__contains__',
'__delattr__',
'__delitem__',
'__dict__',
'__dir__',
'__doc__',
'__eq__',
'__fields__',
'__format__',
'__ge__',
'__getattr__',
'__getattribute__',
'__getitem__',
'__gt__',
'__hash__',
'__init__',
'__iter__',
'__le__',
'__len__',
'__lt__',
'__mappings__',
'__module__',
'__ne__',
'__new__',
'__primary_key__',
'__reduce__',
'__reduce_ex__',
'__repr__',
'__setattr__',
'__setitem__',
'__sizeof__',
'__str__',
'__subclasshook__',
'__table__',
'__weakref__',
'age',
'clear',
'copy',
'find',
'findAll',
'findNumber',
'fromkeys',
'get',
'getValue',
'getValueOrDefault',
'id',
'items',
'keys',
'pop',
'popitem',
'remove',
'save',
'setdefault',
'update',
'values']
In [6]:
In [6]: a.__dict__
Out[6]: mappingproxy({'age': <orm.StringField object at 0x0000000004899A20>, '__
mappings__': {}, '__table__': 'user', '__primary_key__': None, '__module__': '__
main__', '__doc__': None, '__fields__': [], 'id': <orm.StringField object at 0x0
0000000048999E8>})

In [7]: a.__mappings__
Out[7]: {}
　　从这里可以看到，是以{'id':object,'age':object}向上传递到元类的attr，最后在__new__中处理后，重新传递给__init__初始化。