source: django_apps/foobar/models.py

from django.db import models

# Create your models here.
class Role(models.Model):
  name = models.CharField(max_length=50)
  def __unicode__(self):
    return self.name

  def save(self, *args, **kwargs):
    is_new = self.id is None
    super(Role, self).save(*args, **kwargs)
    if is_new:
      for service in Service.objects.all():
        alert = Alert(role=self, service=service, enable=False)
        alert.save()

class Service(models.Model):
  name = models.CharField(max_length=50)
  def __unicode__(self):
    return self.name

  def save(self, *args, **kwargs):
    is_new = self.id is None
    super(Service, self).save(*args, **kwargs)
    if is_new:
      for role in Role.objects.all():
        alert = Alert(role=role, service=self, enable=False)
        alert.save()

class Alert(models.Model):
  enable = models.BooleanField()
  role = models.ForeignKey(Role)
  service = models.ForeignKey(Service)
  class Meta:
    unique_together = (('role', 'service'))