何时使用pre_save，保存，post_save在django？ [英] when to use pre_save, save, post_save in django?

问题描述

我看到，当保存模型实例时，我可以覆盖或定义 pre_save，save，post_save 来执行所需的操作。

I see I can override or define pre_save, save, post_save to do what I want when a model instance gets saved.

哪一种是首选，在哪种情况下为什么？

Which one is preferred in which situation and why?

推荐答案

我将尽力解释一个例子：

I shall try my best to explain it with an example:

pre_save 和 post_save 是信号，由...发送模型。简单来说，在模型的保存之前或之后采取的操作将被调用。

pre_save and post_save are signals that are sent by the model. In simpler words, actions to take before or after the model's save is called.

A 保存 触发以下步骤

• 发出预先保存的信号。


• 预处理数据。


• 大多数字段不进行预处理 - 现场数据保持原样。


• 准备数据库的数据。


• 将数据插入数据库。


• 发出后保存信号。

• Emit a pre-save signal.
• Pre-process the data.
• Most fields do no pre-processing — the field data is kept as-is.
• Prepare the data for the database.
• Insert the data into the database.
• Emit a post-save signal.

Django提供了一种覆盖这些信号的方法。

Django does provide a way to override these signals.

现在，

pre_save 信号可以被覆盖在实际保存到数据库之前进行一些处理 - 例子：（我不知道一个很好的例子，其中 pre_save 将是理想的在我的头顶部）

pre_save signal can be overridden for some processing before the actual save into the database happens - Example: (I dont know a good example of where pre_save would be ideal at the top of my head)

假设您有一个 ModelA ，它存储对 ModelB 其中不已被编辑。为此，您可以在 ModelB pre_save 信号以通知 ModelA c $ c>的保存方法被调用（没有什么阻止你注册一个 post_save 信号在这里）。

Lets say you have a ModelA which stores reference to all the objects of ModelB which have not been edited yet. For this, you can register a pre_save signal to notify ModelA right before ModelB's save method gets called (Nothing stops you from registering a post_save signal here too).

现在，调用模型的保存方法（不是信号） - 默认情况下，每个模型有一个保存方法，但您可以覆盖它：

Now, save method (it is not a signal) of the model is called - By default, every model has a save method, but you can override it:

class ModelB(models.Model):
    def save(self):
        #do some custom processing here: Example: convert Image resolution to a normalized value
        super(ModelB, self).save()

然后，您可以注册 post_save （这更常用于 pre_save ）

Then, you can register the post_save signal (This is more used that pre_save)

一个常见的用例是 UserProfile 对象创建时，用户对象在系统中创建。

A common usecase is UserProfile object creation when User object is created in the system.

您可以注册一个 post_save 信号，它创建一个对应于每个用户的 UserProfile 系统。

You can register a post_save signal which creates a UserProfile object that corresponds to every User in the system.

信号是保持模块化和显式化的一种方式。 （显式通知 ModelA 如果我保存或更改 ModelB ）

Signals are a way to keep things modular, and explicit. (Explicitly notify ModelA if i save or change something in ModelB )

我会想到更具体的现实世界的例子，试图更好地回答这个问题。与此同时，我希望这可以帮助你

I shall think of more concrete realworld examples in an attempt to answer this question better. In the meanwhile, I hope this helps you

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