#34733: m2m_changed signal is unaware if .set() method is being called
-------------------------------------+-------------------------------------
Reporter: lekjos | Owner: lekjos
Type: New feature | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by lekjos):
* owner: nobody => lekjos
* status: new => assigned
Old description:
> When a `related_m2m_field.set()` method is called, the `m2m_changed`
> signal is fired four times for action: `pre_remove`, `post_remove`,
> `pre_add`, and `post_add` (or `pre_clear` / `post_clear`).
>
> This poses a problem if the signal is supposed to run validation on the
> final state of the model. For example, let's say I have a model called
> `Customer` with a many-to-many relation to `SubscriptionPlan` and I have
> a `m2m_changed` signal that is supposed to validate if their
> SubscriptionPlan is valid.
>
> If I call `subscription_plans.set([some_plans])`, the many-to-many
> manager will first call `subscription_plans.remove()` then `.add()`
> inside an atomic transaction. If my signal validates on the first
> `remove()`, it could be in an invalid state, even though it won't be by
> the time the `.add()` completes.
>
> To get around this, I had to create a custom ManyToManyField with a
> custom RelatedManager that set an instance variable on the model when the
> `.set()` method was called. I'd like to propose adding a feature to the
> RelatedManager or to the signals to make the `m2m_changed` signal aware
> of if the `.set()` method was called when running. This could be a
> private attribute on the instance or extra information passed to the
> signal receiver.
>
> If this feature already exists or there's a decent workaround, let me
> know and i'll close the ticket! Otherwise, I have a patch in mind that I
> can raise a PR for and attach.
New description:
When a `related_m2m_field.set()` method is called, the `m2m_changed`
signal is fired four times for action: `pre_remove`, `post_remove`,
`pre_add`, and `post_add` (or `pre_clear` / `post_clear`).
This poses a problem if the signal is supposed to run validation on the
final state of the model. For example, let's say I have a model called
`Customer` with a many-to-many relation to `SubscriptionPlan` and I have a
`m2m_changed` signal that is supposed to validate if their
SubscriptionPlan is valid.
If I call `Customer.subscription_plans.set([some_plans])`, the many-to-
many manager will first call `subscription_plans.remove()` then `.add()`
inside an atomic transaction. If my signal validates on the first
`.remove()`, it could be in an invalid state, even though it won't be by
the time the `.add()` completes. However, I still need to be able to
validate on the signal if the standard `.remove()` method is called.
To get around this, I had to create a custom ManyToManyField with a custom
RelatedManager that set an instance variable on the model when the
`.set()` method was called. I'd like to propose adding a feature to the
RelatedManager or to the signals to make the `m2m_changed` signal aware of
if the `.set()` method was called when running. This could be a private
attribute on the instance or extra information passed to the signal
receiver.
If this feature already exists or there's a decent workaround, let me know
and i'll close the ticket! Otherwise, I have a patch in mind that I can
raise a PR for and attach.
--
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Ticket URL: <https://code.djangoproject.com/ticket/34733#comment:1>
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