Django accessing ManyToMany fields from post_save signal

Question

I have a Django model and I want to modify the object permissions on or just after save. I have tried a few solutions and the post_save signal seemed the best candidate for what I want to do:

    class Project(models.Model):
        title = models.CharField(max_length=755, default='default')
        assigned_to = models.ManyToManyField(
            User, default=None, blank=True, null=True
        )
        created_by = models.ForeignKey(
            User,
            related_name="%(app_label)s_%(class)s_related"
        )


    @receiver(post_save, sender=Project)
    def assign_project_perms(sender, instance, **kwargs):
        print("instance title: "+str(instance.title))
        print("instance assigned_to: "+str(instance.assigned_to.all()))

In this case, when a Project is created, the signal fires and I see the title, but an empty list for the assigned_to field.

How can I access the saved assigned_to data following save?

Answer 1

You're not going to. M2Ms are saved after instances are saved and thus there won't be any record at all of the m2m updates. Further issues (even if you solve that) are that you're still in a transaction and querying the DB won't get you m2m with proper states anyways.

The solution is to hook into the m2m_changed signal instead of post_save.

https://docs.djangoproject.com/en/dev/ref/signals/#m2m-changed

Your sender then would be Project.assigned_to.through