Feature Request: Add through_defaults as argument for m2m_changed signal

[Shmuel Treiger]

Use case:

In English:
I have many policies, each of which has a status from a table of statuses and is not unique to that policy. I associate these policies to my many domains through the many_to_many table. Domains can have many policies, but only one of each type of policy. Likewise, the same policy can apply to multiple domains. (In practice, there are a few thousand domains and a dozen or so policies.) In the following code, I have successfully implemented this relationship at the database level.

In pseudocode:

Domain(models.Model) <fields> PolicyStatus(models.Model): status = models.CharField(<>) Policy(models.Model) status = models.ForeignKey(PolicyStatus, <>) domains = models.ManyToManyField(Domain, through="Domain_Policy_m2m", <>) <fields> Domain_Policy_m2m(models.Model): class Meta: constraints = [ models.UniqueConstraint( fields=["domain", "status", ], name="unique_constraint" ) ] domain = models.ForeignKey(Domain, <>) policy = models.ForeignKey(Policy, <>) status = models.ForeignKey(PolicyStatus, <>)

Scenario:
I go to associate a new policy with a domain:
domain.policy_set.add(policy, through_defaults={"status": policy.status})

However, the domain already has a policy with this status and raises IntegrityError. Instead, I would like to replace the existing policy with the new one. I can do this in my code:

new_policy = Policy(<>) existing_policy = domain.policy_set.filter(status=new_policy.status).first() if existing_policy: domain.policy_set.remove(existing_policy) domain.policy_set.add(policy, through_defaults={"status": policy.status})

Instead, I wanted to take care of this in a more permanent way, so I created a m2m_change signal receiver, per the documentation:

from <> import Policy @receiver(m2m_changed, sender=Domain_Policy_m2m) def delete_old_m2m(action, instance, pk_set, **kwargs): if action == "pre_add": pk = min(pk_set) policy = Policy.objects.get(pk=pk) status = policy.status existing_policy = Domain_Policy_m2m.objects.filter(status=status, domain=instance) if existing_policy.exists(): existing_policy.delete()

This works, but it's not pretty. It would be excellent if I could have the through_defaults as an argument, so I could do something like:

@receiver(m2m_changed, sender=Domain_Policy_m2m) def delete_old_m2m(action, instance, through_defaults, **kwargs): if action == "pre_add": status = through_defaults["status"] existing_policy = Domain_Policy_m2m.objects.filter(status=status, domain=instance) if existing_policy.exists(): existing_policy.delete()

Even the object(s) being added would be so much more convenient than the pk:

@receiver(m2m_changed, sender=Domain_Policy_m2m) def delete_old_m2m(action, instance, model_set, **kwargs): if action == "pre_add": model = min(model_set) existing_policy = Domain_Policy_m2m.objects.filter(status=model.status, domain=instance) if existing_policy.exists(): existing_policy.delete()
I could alternatively override Domain_Policy_m2m.save(<>) and use Domain_Policy_m2m.objects.create(<>) but that's just seems less easily maintainable for future developers.

I'll also note that the Django documentation (linked above and here) indicates that this would be incorrect implementation:

Using add() with a many-to-many relationship, however, will not call any save() methods (the bulk argument doesn’t exist), but rather create the relationships using QuerySet.bulk_create(). If you need to execute some custom logic when a relationship is created, listen to the m2m_changed signal, which will trigger pre_add and post_add actions.


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