metaclass實現orm的原理
阿新 • • 發佈:2018-12-21
需要從呼叫去入手,理解框架為什麼這麼寫,另外,在原文基礎上,修改了Model程式碼,原文有些做複雜了。
另外最重要的是,你需要在原文的引導下,自己親自動手實現一遍才能真正理解。
期望實現這樣一個orm框架的呼叫:
class User(Model):
# 定義類的屬性到列的對映:
id = IntegerField('id')
name = StringField('username')
email = StringField('email')
password = StringField('password')
u = User(id=12345, name='Michael', email=' [email protected]', password='my-pwd')
u.save()
輸出:
Found model: User
Found mapping id --> <IntegerField,id>
Found mapping name --> <StringField,username>
Found mapping email --> <StringField,email>
Found mapping password --> <StringField,password>
SQL: insert into User (id,username,email,password) values (?,?,?,?)
ARGS: [12345, 'Michael', ' [email protected]', 'my-pwd']
框架實現:
class Field(object): #儲存資料庫表的欄位名和欄位型別 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) #打印出 <Filed,欄位名> class StringField(Field): def __init__(self, name): return super(StringField, self).__init__(name, 'varchar(100)') class IntegerField(Field): def __init__(self, name): return super().__init__(name, 'bigint') #不同的獲取父類Field的方法
class ModelMetaclass(type):
def __new__(cls, name, bases, attrs):
if name == 'Model': #只對Model的子類起作用,Model不需要去管它
return type.__new__(cls, name, bases, attrs)
print('Found model:',name)
mappings = dict()
for k, v in attrs.items(): #找出屬性中是Field的屬性
if isinstance(v, Field):
print('Found mapping %s --> %s' % (k, v))
mappings[k] = v
for k in mappings.keys(): #從類中刪除在前面找到的是對映關係的靜態屬性
attrs.pop(k)
attrs['__mappings__'] = mappings
attrs['__table__'] = name #假設表名與類名一致
return type.__new__(cls, name, bases, attrs)
class Model(dict, metaclass=ModelMetaclass): #Model是一個dict
def save(self):
fields = []
params = []
args = []
for k, v in self.__mappings__.items():
fields.append(v.name)
params.append('?')
args.append(self[k])
sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(params))
print('SQL: %s' % sql)
print('ARGS: %s' % str(args))
原文獲取欄位引數的方法是使用 getattr方法,但是python的字典是沒有實現這個方法的,所以我們不如直接用字典提供的方法[]去獲取即可