Add "intermediary" kwarg to ModelForm._save_m2m

[Douglas Denhartog]

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.

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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.