#33369: Add through_defaults as argument for m2m_changed signal
-------------------------------------+-------------------------------------
Reporter: | Owner: nobody
ShmuelTreiger |
Type: New | Status: new
feature |
Component: Database | Version: 4.0
layer (models, ORM) |
Severity: Normal | Keywords: m2m_changed
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
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 receive:
{{{
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()
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/33369>
Django <https://code.djangoproject.com/>
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