How to add permissions in Django post_save signal?

Question

I have a problem to add permissions in Django post_save signal because the m2m relationships don't persist. They are correctly displayed with user_permissions.all() after assignment, but as soon as I save the model again the queryset get empty. What am'I doing wrong ?

I don't have the same problem if I assign permissions at the object level with guardian.

models.py

class User(AbstractBaseUser, PermissionsMixin):

    username = models.CharField(db_index=True, max_length=255, unique=True, null=True, blank=True)
    last_name = models.CharField(max_length=255, null=True, blank=True)
    first_name = models.CharField(max_length=255, null=True, blank=True)
    email = models.EmailField(db_index=True, unique=True)    

    def save(self, *args, **kwargs):
        return super(User, self).save(*args, **kwargs)

class Manager(User):
    ...

    def save(self, *args, **kwargs):
        return super(Manager, self).save(*args, **kwargs)

signals.py

@receiver(post_save, sender=Manager)
def assign_manager_permissions(sender, instance, created, raw, using, **kwargs):

    content_type = ContentType.objects.get_for_model(Manager)
        
    print('Current permissions:')    
    for p in instance.user_permissions.all():
        print(p)

    for codename in ['view_manager',
                         'change_manager',
                         'add_manager',
                         'delete_manager']:

        permission = Permission.objects.get(content_type=content_type, codename=codename)
        instance.user_permissions.add(permission)


    instance.refresh_from_db()
    
    print('After refresh permissions:')
    for p in instance.user_permissions.all():
        print(p)

When I save the model the result is always the same:

django-1  | Current permissions:
django-1  | 
django-1  | After refresh permissions:
django-1  | user | manager | Can add manager
django-1  | user | manager | Can change manager
django-1  | user | manager | Can delete manager
django-1  | user | manager | Can view manager

score 0 · Accepted Answer · answered Dec 23 '22 at 21:10

I found the answer to my own question by using transaction.commit, as proposed here.

from django.db import transaction


def on_transaction_commit(func):
    def inner(*args, **kwargs):
        transaction.on_commit(lambda: func(*args, **kwargs))

    return inner


@receiver(post_save, sender=Manager)
@on_transaction_commit
def assign_manager_permissions(sender, instance, created, raw, using, **kwargs):

    # code here

How to add permissions in Django post_save signal?

1 Answers1