#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 | 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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This is my first ticket, so while I attempted to follow the applicable
guidelines, I apologize in advance if this ticket is not a model
guideline-following example.
----
Django's documentation clearly explain Intermediary Model limitations:
"[u]nlike normal many-to-many fields, you can’t use add(), create(), or
set() to create relationships"
Thus, when `ModelForm.save()` is called and an intermediary model field is
not in `self._meta.exclude`, users will get an error like so:
{{{
Cannot set values on a ManyToManyField which specifies an intermediary
model. Use app.IntermediaryModelName's Manager instead.
}}}
This presents complexity when the intermediary field is included in the
form.
My proposal is to add a kwarg `intermediary` to `ModelForm._save_m2m`.
This kwarg behaves exactly like `self._meta.exclude` within
`ModelForm._save_m2m`.
I have already created a github branch with my solution, but have not
created a pull request per contribution guidelines (but will do so if/when
this ticket is approved). My solution adds three/four (3/4) lines of code,
denoted in my solution below with EOL comment `# ADDED`. I believe my
simple solution is more appropriate than a complex introspective analysis
of every potential ManyToManyField's 'through' attribute.
The final code of my proposal is (in django.forms.models):
{{{
def _save_m2m(self, intermediary=None): # ADDED
"""
Save the many-to-many fields and generic relations for this form.
"""
# could assert/convert intermediary == str, list, tuple # ADDED
cleaned_data = self.cleaned_data
exclude = self._meta.exclude
fields = self._meta.fields
opts = self.instance._meta
# Note that for historical reasons we want to include also
# private_fields here. (GenericRelation was previously a fake
# m2m field).
for f in chain(opts.many_to_many, opts.private_fields):
if not hasattr(f, 'save_form_data'):
continue
if fields and f.name not in fields:
continue
if exclude and f.name in exclude:
continue
if intermediary and f.name in intermediary: # ADDED
continue # ADDED
if f.name in cleaned_data:
f.save_form_data(self.instance, cleaned_data[f.name])
}}}
And yes, this means adding the `intermediary` kwarg to `ModelForm.save()`
so that `intermediary` can be passed to `ModelForm._save_m2m`.
--
Ticket URL: <https://code.djangoproject.com/ticket/29346>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
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