使用Django的m2m_changed来修改pre_add的保存内容

Question

我对Django的信号不是很熟悉,可以使用一些帮助.

如何在保存实例之前修改pk_set？我是否必须向信号呼叫者返回一些东西(比如kwargs)？或者我保存instance自己？

作为一个简单的例子:我希望确保在pk=1保存时我的所有视频都包含类别.我该怎么做m2m_changed？

class Video(models.Model):
    category = models.ManyToManyField('Category')

def video_category_changed(sender, **kwargs):
    action = kwargs.pop('action', None)
    pk_set = kwargs.pop('pk_set', None)
    instance = kwargs.pop('instance', None)

    if action == "pre_add":
        if 1 not in pk_set:
            pk_set.update( [ 1 ] )  # adding this to the set
            # do something else?
            # profit?

m2m_changed.connect( video_category_changed, sender=Video.category.through )

Answer 1

只需更新即可pk_set.你不需要做任何额外的工作.保存视频实例后,它将具有pk = 1的类别.

from django.db import models
from django.db.models.signals import m2m_changed
from django.dispatch import receiver

class Category(models.Model):
    pass

class Video(models.Model):
    category = models.ManyToManyField('Category')

@receiver(m2m_changed, sender=Video.category.through)
def video_category_changed(sender, **kwargs):
    action = kwargs.pop('action', None)
    pk_set = kwargs.pop('pk_set', None)    
    if action == "pre_add":
        if 1 not in pk_set:
            pk_set.update([1])

在上述方法中,只有在保存视频实例后才会保存类别.如果要在m2m_changed实例中明确地保存它们,也可以按如下方式执行此操作.

@receiver(m2m_changed, sender=Video.category.through)
def video_category_changed(sender, **kwargs):
    instance = kwargs.pop('instance', None)
    pk_set = kwargs.pop('pk_set', None)
    action = kwargs.pop('action', None)
    if action == "pre_add":
        if 1 not in pk_set:
            c = Category.objects.get(pk=1)
            instance.category.add(c)
            instance.save()