#12203: ManyToManyField with through model can't be used in admin
-------------------------------------+-------------------------------------
Reporter: David Gouldin | Owner: nobody
Type: Bug | Status: new
Component: contrib.admin | Version: master
Severity: Normal | Resolution:
Keywords: M2M, admin, | Triage Stage: Accepted
through, through_fields |
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Dennis):
Replying to [comment:22 Dmitry Mugtasimov]:
The trick of setting `JobTitleExperienceThrough._meta.auto_created = True`
in `formfield_for_manytomany` does indeed enable the
`ModelMultipleChoiceField` on the admin page without causing migration
issues (as far as I can see).
However, there is a dangerous side-effect of using this, in case you have
other models with a "cascading" relation directly to your explicit
through-model, e.g. `models.ForeignKey(to=JobTitleExperienceThrough,
on_delete=models.CASCADE)`:
If the initial queryset for the m2m field `JobTitle.experiences` is
filtered (e.g. as in the docs
[https://docs.djangoproject.com/en/3.0/ref/contrib/admin/#django.contrib.admin.ModelAdmin.formfield_for_manytomany]),
and, for whatever reason, the filter excludes some `Experience` objects
that have previously been assigned, then the corresponding records from
`JobTitleExperienceThrough` will be silently deleted, including any
records for other models pointing to them.
This means there is a serious potential for "silent" data loss.
This is because the many-to-many relation will now be updated using
`ManyRelatedManager.set()` (via `ModelAdmin.save_related()` ->
`form.save_m2m()` -> `BaseModelForm._save_m2m()` ->
`ManyToManyField.save_form_data()`).
Silent deletion could be prevented by setting `on_delete=models.PROTECT`
on any relation to the explicit through-model, but then you would first
have to be aware of the necessity.
When using a "real" auto-created `through` table (i.e. an implicit one)
the issue does not arise, because there is no way to set a `ForeignKey` to
the implicit `through`, as far as I know.
When using an inline for `JobTitleExperienceThrough`, this issue does not
occur.
--
Ticket URL: <https://code.djangoproject.com/ticket/12203#comment:23>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-updates/066.794eb1972440844545cc2226e74dc6c6%40djangoproject.com.
