m2m_changed signal registration

Hi,

I am having trouble registering a function to the m2m_changed signal. My code is as follows:

from django.db.models.signals import m2m_changed
from django.db import models
from django.dispatch.dispatcher import receiver
     
class Project(models.Model):
    employees = models.ManyToManyField('staff.Person', through='project.PersonProjectMembership',
    related_name='projects')
     
class PersonProjectMembership(models.Model):
    project = models.ForeignKey('project.Project', related_name="person_memberships")
    person = models.ForeignKey('staff.Person', related_name="project_memberships")
    lead = models.BooleanField(default=False)
    position = models.CharField(max_length=50)
    project_manager = models.BooleanField(
        default=False
    )
     
    class Meta:
        permissions = (
            ('view_personprojectmembership', _('View person project membership')),
        )
     
@receiver(m2m_changed, sender=Project.employees.through)
def _on_save_project_assign_privileges(sender, instance, action, reverse, model, pk_set, using, **kwargs):
    # [...]

Somehow it doesn't seem to work, the execution point never reaches the _on_save_project_assign_privileges function when I modify the m2m relation (I am using the admin interface for that). On the other side, I have another method registered to the post_save signal for the same object and it works correctly.

I have read the signals documentation -concretely the m2m_changed-, re-read my code, googled for it, asked in other sites... but I have not been able to find a solution. Any help please? Thanks!

Roberto