需要从调用去入手,理解框架为什么这么写,另外,在原文基础上,修改了Model代码,原文有些做复杂了。
另外最重要的是,你需要在原文的引导下,自己亲自动手实现一遍才能真正理解。
期望实现这样一个orm框架的调用:
class User(Model):
# 定义类的属性到列的映射:
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()
输出:
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']
框架实现:
class Field(object): #保存数据库表的字段名和字段类型
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,字段名>
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') #不同的获取父类Field的方法
class ModelMetaclass(type):
def __new__(cls, name, bases, attrs):
if name == 'Model': #只对Model的子类起作用,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(): #找出属性中是Field的属性
if isinstance(v, Field):
print('Found mapping %s --> %s' % (k, v))
mappings[k] = v
for k in mappings.keys(): #从类中删除在前面找到的是映射关系的静态属性
attrs.pop(k)
attrs['__mappings__'] = mappings
attrs['__table__'] = name #假设表名与类名一致
return type.__new__(cls, name, bases, attrs)
class Model(dict, metaclass=ModelMetaclass): #Model是一个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))
原文获取字段参数的方法是使用 getattr方法,但是python的字典是没有实现这个方法的,所以我们不如直接用字典提供的方法[]去获取即可