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利用django model save方法對(duì)未更改的字段依然進(jìn)行了保存

更新時(shí)間：2020年03月28日 16:35:25 作者：寒木

這篇文章主要介紹了利用django model save方法對(duì)未更改的字段依然進(jìn)行了保存，具有很好的參考價(jià)值，希望對(duì)大家有所幫助。一起跟隨小編過(guò)來(lái)看看吧

我就廢話不多說(shuō)了，直接上代碼吧！

 
obj = Obj.objects.get(id=1)
print obj.name #此時(shí)name的值假定為'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

最終的name結(jié)果依然為'abc'。save()保存時(shí),雖然沒(méi)有更改其它字段，但依然會(huì)將內(nèi)存中的值，再次存入數(shù)據(jù)庫(kù)，子函數(shù)和其它進(jìn)程更改的值會(huì)被覆蓋。

補(bǔ)充知識(shí)：Django Signals之pre_save & post_save ，pre_delete & post_delete

Listening to signals（信號(hào)監(jiān)聽(tīng)）

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

（為了接收信號(hào)，需要使用Signal.connect()方法注冊(cè)一個(gè)接收器函數(shù)，當(dāng)信號(hào)發(fā)送后接收器函數(shù)就會(huì)被調(diào)用）

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

Parameters: 參數(shù)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.

一般使用是這樣的：

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

#Connecting receiver functions（連接接收器函數(shù)）
#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)

當(dāng)然，喜歡裝逼（高技術(shù)）的人喜歡使用另外一種方式，receiver()裝飾器：

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

下面將針對(duì)本文標(biāo)題來(lái)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.

在模型保存操作執(zhí)行前或者執(zhí)行后發(fā)送信號(hào)

Connecting to signals sent by specific senders

（連接到特定發(fā)送器發(fā)送的信號(hào)）

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.

（有些信號(hào)會(huì)被多次發(fā)送，但是我們通常只是對(duì)其中的一些信號(hào)子集感興趣，下面將演示針對(duì)具體的某個(gè)模型的pre_save以及post_save來(lái)發(fā)送信號(hào)）

從上邊的運(yùn)行結(jié)果可以看出，兩個(gè)函數(shù)都被執(zhí)行了，但是是有一定的執(zhí)行順序的，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.

在模型刪除操作執(zhí)行前或者執(zhí)行后發(fā)送信號(hào)

下面將演示pre_delete與post_delete這兩個(gè)模型信號(hào)的使用