怎么利用django model save方法對未更改的字段進行保存

小編給大家分享一下怎么利用django model save方法對未更改的字段進行保存，相信大部分人都還不怎么了解，因此分享這篇文章給大家參考一下，希望大家閱讀完這篇文章后大有收獲，下面讓我們一起去了解一下吧！

直接上代碼吧！

 obj = Obj.objects.get(id=1)
print obj.name #此時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結果依然為'abc'。save()保存時,雖然沒有更改其它字段，但依然會將內存中的值，再次存入數據庫，子函數和其它進程更改的值會被覆蓋。

補充知識：Django Signals之pre_save & post_save ，pre_delete & post_delete

Listening to signals（信號監聽）

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，需要使用Signal.connect()方法注冊一個接收器函數，當信號發送后接收器函數就會被調用）

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

Parameters: 參數解析

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）
#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)

當然，喜歡裝逼（高技術）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.

下面將針對本文標題來做示例：

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

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

在模型保存操作執行前或者執行后發送信號

Connecting to signals sent by specific senders

（連接到特定發送器發送的信號）

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.

（有些信號會被多次發送，但是我們通常只是對其中的一些信號子集感興趣，下面將演示針對具體的某個模型的pre_save以及post_save來發送信號）

從上邊的運行結果可以看出，兩個函數都被執行了，但是是有一定的執行順序的，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.

在模型刪除操作執行前或者執行后發送信號

下面將演示pre_delete與post_delete這兩個模型信號的使用

和save的運行邏輯一樣，pre信號先觸發，post后觸發

以上是“怎么利用django model save方法對未更改的字段進行保存”這篇文章的所有內容，感謝各位的閱讀！相信大家都有了一定的了解，希望分享的內容對大家有所幫助，如果還想學習更多知識，歡迎關注億速云行業資訊頻道！