#29346: Add "intermediary" kwarg to ModelForm._save_m2m
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Reporter: Douglas Denhartog | Owner: nobody
Type: New feature | Status: new
Component: Forms | Version: 2.0
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
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Comment (by Douglas Denhartog):
Replying to [comment:4 Tim Graham]:
> Sorry, what I meant was, why isn't `Meta.exclude` sufficient for this
use case. If you added a test to your branch that could demonstrate the
use case.
>
> Looking at the existing implementation of `_save_m2m()`, a possible
alternative (that doesn't require changes to Django) could be to remove
the field from `self.cleaned_data` after you save the intermediate model
manually.
**To specifically answer your question**: `Meta.exclude` is not sufficient
when someone wants to **include** a model intermediary many-to-many field
in a ModelForm. If one uses `Meta.exclude`, per the Django docs, one must
"manually add the excluded fields back to the form, [and the field] will
not be initialized from the model instance." Further, as you mentioned,
without my solution, one must **also** manipulate `self.cleaned_data`.
Thus, without my solution, one must:
(1) manually add the intermediate data to the form instance (vs. my
solution where Django does this itself);
(2) manually add the form field (both solutions require);
(3) (within `ModelForm.save()`) remove the field from cleaned_data, then
`super().save()`, then add the field back into `self.cleaned_data` (vs. my
solution where this is unnecessary).
I'll build some tests to demonstrate (will be a couple of days until I can
do so).
--
Ticket URL: <https://code.djangoproject.com/ticket/29346#comment:5>
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