Re: [Django] #33369: Add through_defaults as argument for m2m_changed signal

[Django]

#33369: Add through_defaults as argument for m2m_changed signal
-+-
 Reporter:  ShmuelTreiger|Owner:  nobody
 Type:  New feature  |   Status:  closed
Component:  Database layer   |  Version:  dev
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  wontfix
 Keywords:  m2m_changed  | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Mariusz Felisiak):

 You can start a discussion on DevelopersMailingList if you don't agree,
 where you'll reach a wider audience and see what other think, and
 [https://docs.djangoproject.com/en/stable/internals/contributing/bugs-and-
 features/#requesting-features follow the guidelines with regards to
 requesting features].

 Also, please don't use offensive language, and be aware that
 [https://www.djangoproject.com/conduct/ "Code of Conduct"] applies to the
 Trac.

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Re: [Django] #33369: Add through_defaults as argument for m2m_changed signal

[Django]

#33369: Add through_defaults as argument for m2m_changed signal
-+-
 Reporter:  ShmuelTreiger|Owner:  nobody
 Type:  New feature  |   Status:  closed
Component:  Database layer   |  Version:  dev
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  wontfix
 Keywords:  m2m_changed  | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by ShmuelTreiger):

 I'm literally doing what your documentation tells me to do, and it's
 shitty, hacky, and horrible. Your response is "Won't fix". Excellent.

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Re: [Django] #33369: Add through_defaults as argument for m2m_changed signal

[Django]

#33369: Add through_defaults as argument for m2m_changed signal
-+-
 Reporter:  ShmuelTreiger|Owner:  nobody
 Type:  New feature  |   Status:  closed
Component:  Database layer   |  Version:  dev
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  wontfix
 Keywords:  m2m_changed  | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Mariusz Felisiak):

 * status:  new => closed
 * resolution:   => wontfix


Comment:

 Please don't reopen closed ticket. You have a custom intermediate models,
 so in your case `.create()` or `.save()` seem ideal to handle this logic.
 As you already described there are at least two ways for your use case. We
 don't want to add a third way. Moreover, signals are required in really
 rare cases and it's definitely not one of them.

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Re: [Django] #33369: Add through_defaults as argument for m2m_changed signal

[Django]

#33369: Add through_defaults as argument for m2m_changed signal
-+-
 Reporter:  ShmuelTreiger|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  dev
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  m2m_changed  | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by ShmuelTreiger):

 * status:  closed => new
 * resolution:  invalid =>


Comment:

 `domain.policy_set.add(<>)` does not call save() on the m2m model. From
 the documentation here
 
https://docs.djangoproject.com/en/4.0/ref/models/relations/#django.db.models.fields.related.RelatedManager.add

 >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.

 I tried `save`, it didn't work, so I found the documentation which told me
 to use `m2m_changed`.

 I suppose instead of `add`, I could do
 `Domain_Policy_m2m.objects.create(<>)` but that's just seems less than
 ideal.

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Re: [Django] #33369: Add through_defaults as argument for m2m_changed signal

[Django]

#33369: Add through_defaults as argument for m2m_changed signal
-+-
 Reporter:  ShmuelTreiger|Owner:  nobody
 Type:  New feature  |   Status:  closed
Component:  Database layer   |  Version:  dev
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  invalid
 Keywords:  m2m_changed  | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Mariusz Felisiak):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 Thanks for the report , however the `m2m_changed` signal was created for
 automatically created intermediate models. If you have an explicit
 `through` model, you can attached `Model` signals to it, i.e. `pre_save`,
 `post_save`, `pre_delete`, and `post_delete`. Personally, I would use add
 this logic to the
 
[https://docs.djangoproject.com/en/4.0/ref/models/instances/#django.db.models.Model.save
 Domain_Policy_m2m.save()] and don't use signals at all, this is not a
 situation where signals are needed. If have further questions, see
 TicketClosingReasons/UseSupportChannels for ways to get help.

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Re: [Django] #33369: Add through_defaults as argument for m2m_changed signal

[Django]

#33369: Add through_defaults as argument for m2m_changed signal
-+-
 Reporter:  ShmuelTreiger|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  dev
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  m2m_changed  | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by ShmuelTreiger):

 * version:  4.0 => dev


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[Django] #33369: Add through_defaults as argument for m2m_changed signal

[Django]

#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)
 

 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 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()
 }}}

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