title: Django accessing ManyToMany fields from post_save signal
body: <p>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 <code>post_save</code> signal seemed the best candidate for what I want to do:</p>
  
  <pre><code>    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()))
  </code></pre>
  
  <p>In this case, when a Project is created, the signal fires and I see the <code>title</code>, but an empty list for the <code>assigned_to</code> field. </p>
  
  <p>How can I access the saved <code>assigned_to</code> data following save?</p>