利用django model save方法对未更改的字段依然进行了保存

我就废话不多说了，直接上代码吧！

obj = Obj.objects.get(id=1)
print obj.name #此时name的值假定为'abc'
 
def handler(oid):
 obj = Obj.objects.get(id=oid)
 obj.name = '123'
 obj.save()
handler(obj.id)
obj.age = 10
obj.save()
print obj.name

最终的name结果依然为'abc'。save()保存时,虽然没有更改其它字段，但依然会将内存中的值，再次存入数据库，子函数和其它进程更改的值会被覆盖。

补充知识：Django Signals之pre_save & post_save ，pre_delete & post_delete

Listening to signals（信号监听）

To receive a signal, register a receiver function using the Signal.connect() method. The receiver function is called when the signal is sent.

（为了接收信号，需要使用Signal.connect()方法注册一个接收器函数，当信号发送后接收器函数就会被调用）

Signal.connect(receiver, sender=None, weak=True, dispatch_uid=None)[source]

Parameters: 参数解析

receiver ? The callback function which will be connected to this signal. See Receiver functions for more information.

sender ? Specifies a particular sender to receive signals from. See Connecting to signals sent by specific senders for more information.

weak ? Django stores signal handlers as weak references by default. Thus, if your receiver is a local function, it may be garbage collected. To prevent this, pass weak=False when you call the signal's connect() method.

dispatch_uid ? A unique identifier for a signal receiver in cases where duplicate signals may be sent. See Preventing duplicate signals for more information.

一般使用是这样的：

def my_callback(sender, **kwargs):
 print("Request finished!")

#Connecting receiver functions（连接接收器函数）
#There are two ways you can connect a receiver to a signal. You can take the manual connect route:

from django.core.signals import request_finished

request_finished.connect(my_callback)

当然，喜欢装逼（高技术）的人喜欢使用另外一种方式，receiver()装饰器：

receiver(signal)[source]#用法
Parameters: signal ? A signal or a list of signals to connect a function to.

下面将针对本文标题来做示例：

django.db.models.signals.pre_save & django.db.models.signals.post_save

Sent before or after a model's save() method is called.

在模型保存操作执行前或者执行后发送信号

Connecting to signals sent by specific senders

（连接到特定发送器发送的信号）

Some signals get sent many times, but you'll only be interested in receiving a certain subset of those signals. For example, consider the django.db.models.signals.pre_save signal sent before a model gets saved. Most of the time, you don't need to know when any model gets saved ? just when one specific model is saved.

（有些信号会被多次发送，但是我们通常只是对其中的一些信号子集感兴趣，下面将演示针对具体的某个模型的pre_save以及post_save来发送信号）

从上边的运行结果可以看出，两个函数都被执行了，但是是有一定的执行顺序的，pre then post

In these cases, you can register to receive signals sent only by particular senders. In the case of django.db.models.signals.pre_save, the sender will be the model class being saved, so you can indicate that you only want signals sent by some model:

django.db.models.signals.pre_delete & django.db.models.signals.post_delete

Sent before or after a model's delete() method or queryset's delete() method is called.

在模型删除操作执行前或者执行后发送信号

下面将演示pre_delete与post_delete这两个模型信号的使用

和save的运行逻辑一样，pre信号先触发，post后触发

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