provide declarative syntax to define FormSets - including ModelFormSet and InlineFormSet

[Koen Biermans <koen.biermans@…>]

Provide a declarative mechanism to define modelformsets or inlineformsets.

The attached patch allows definitions like this:

class AuthorForm(forms.ModelForm):
    class Meta:
        model = Author

class DeclarativeAuthorFormSet(forms.models.ModelFormSet):
    form = AuthorForm
    model = Author
    extra = 3

and

class BookForm(forms.ModelForm):
    class Meta:
        model = Book

class DeclarativeAuthorBooksFormSet(forms.models.InlineFormSet):
    model = Book
    form = BookForm
    parent = 'author'

An advantage is that the defined form is directly used as the form class in the formset, not as a base class for a new form class (what inlineformset_factory does). This way specific field definitions and other customisations in the form work like they should so you don't need to redefine things in init().

[Koen Biermans <koen.biermans@…>]

patch to provide declarative syntax for modelformset and inlineformset

[Stephen Burrows]

Closed #16289 as a duplicate - it proposed essentially the same thing as this ticket, but included all FormSets, not just ModelFormSets. Hence I'm widening the scope of this a little. (Also marking as needs tests since the tests in the patch are doctests.)

[Bas Peschier]

Ok, so I missed this one O:)

I disagree with the implementation in the patch, no MetaClass is needed and just simple additions to the class definitions to make it work, maybe with the exception of InlineFormSet. Will attach a patch later on.

[rasca]

Adding a comment from #16289:

This would be very useful for a nicer implementation of the formset derived cbv #16256.

The BaseInlineFormSet? is the only tricky one with the _get_foreign_key() and the max_num override when fk.unique=True .

I'd also move the defaults (e.g. extra=3) to the BaseFormSet? and make the factories only pass dict keys whose value isn't None to type().

[Carl Meyer]

The current factory-function implementation of formsets is unnecessarily confusing to new users, and unlike anything else in Django. It can also cause obscure issues with things that should "just work" (see e.g. ​http://stackoverflow.com/questions/622982/django-passing-custom-form-parameters-to-formset/624013#624013). It's ironic that we provide declarative syntax for things like models and forms where it requires metaclass magic, but in the case of formsets where a declarative syntax is a simple matter of normal Python subclassing, we don't :-)

Haven't reviewed the latest patch in depth, but the broad strokes look correct to me.

[Tim Graham]

I don't know that deprecating the factory functions (at least immediately) is a good idea. This could cause a lot of annoying churn for large projects.

[Parth Patil]

Here is a link to the pull request. This is the first iteration of the patch so may require some critical review.
​https://github.com/django/django/pull/11481

[Asif Saifuddin Auvi]

Parth, are you willing to complete this for 3.1?

[Mitchina]

I opened a PR (still in draft) to continue the work initiated by Parth Patil on: ​https://github.com/django/django/pull/17905

[Sarah Boyce]

Left a small review, please update "Patch needs improvement" to False when these changes are applied and I will review more deeply 👍