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This article will explain in detail how to use signals to monitor changes in the field values of Django model objects. The content of the article is of high quality, so the editor shares it for you as a reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

The function of Django signal (Signals) is similar to the action of WordPress, which is used to add the broadcast (dispatch) and receiving (receive) mechanism of events to the project globally. Among them, the flexible use of its built-in model signal (Model Signals) reception function can monitor the changes of most model objects (Model instances). Because there is no need to modify the code of the model itself, it has the advantage of low coupling in cross-application (App) monitoring.

Basic usage

The basic usage of signals is described in detail in the topics and references in the official documentation. This article only mentions a few main points (this article environment: Django 1.8 & Python 3.4):

Code organization

It is officially recommended to add a signals.py file to the application directory, and refer to the custom application configuration (AppConfig) in the application configuration section of the official document, overload the run method of the application configuration class, and call from within this method. Import signals

Receive signal

It is recommended to use django.dispatch.receiver as a decorator for signal reception:

From django.db.models import signals

From django.dispatch import receiver

From students.models import Student

From .models import Announcement

@ receiver (signals.post_save, sender=Student)

Def welcome_student (instance, created, * * kwargs):

If created:

Announcement.objects.create (content='Welcome new student'+ instance.name)

In terms of code readability, it is recommended that a receiver function do only one thing.

Monitor changes in specific field (field) values

As you can see from the previous code, by receiving the model post_save signal, you can know that the operation of saving the model object has taken place, and you can tell whether the model object is created or updated. However, the model signal does not provide a broadcast function for a specific field value change. Although the signal provides a update_fields parameter, it does not prove that the field value of the field name in this parameter must have changed, so we should adopt a workaround method combined with post_init signal.

To give an example: make an announcement when a student's name changes.

From django.db.models import signals

From django.dispatch import receiver

From students.models import Student

From .models import Announcement

@ receiver (signals.post_init, sender=Student)

Def welcome_student (instance, * * kwargs):

Instance.__original_name = instance.name

@ receiver (signals.post_save, sender=Student)

Def welcome_student (instance, created, * * kwargs):

If not created and instance.__original_name! = instance.name:

Announcement.objects.create (content=

'Student% s has renamed to% s'% (instance.__original_name, instance.name))

To put it simply, when the model broadcasts the post_init signal, the current field value is cached in the model object; when the model broadcasts post_save (or pre_save), the current field value of the model object is compared with the cached field value, and if it is different, the field value is considered to have changed.

On how to use signal monitoring Django model object field value changes are shared here, I hope the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

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