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) PolicyStatus(models.Model): status =
models.CharField(<>) Policy(models.Model) status =
models.ForeignKey(PolicyStatus, <>) domains =
models.ManyToManyField(Domain, through="Domain_Policy_m2m", <>)
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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