在post_save上添加M2M

Question

I have an object that has a m2m relation, and I would like to populate it after saving.

The problem is that the signal is triggered, but the command add doesn't work. I did try the same steps using python shell, and it worked fine.

class Event(models.Model):
  name = models.CharField(max_lenght=40)
  location = models.ManyToManyField('Location')

class Location(models.Model):
   address = models.CharField(max_lenght=60)


@receiver(post_save, sender=Event)
def populate_location(sender, instance, **kwargs):
   instance.locations.add(*Locations.objects.all())

Any hint?

Answer 1

If you want to add all the objects you can use set() instead of add , you can have a look on documentation - https://docs.djangoproject.com/en/2.1/ref/models/relations/#django.db.models.fields.related.RelatedManager.set

One more suggestion, if you are using add then try to print instance.locations just after executing - instance.locations.add(*Locations.objects.all()) and post the result.

Answer 2

I have found the solution. I forgot to mention that I was trying to save from admin, and it seems that it was a crutial detail, sorry about this.

https://timonweb.com/posts/many-to-many-field-save-method-and-the-django-admin/