Add a hook to customize the admin's formsets parameters

[David Sanders]

New feature that adds a method on InlineModelAdmin for providing initial data for the inline formset. By default there is no implementation, although one could be implemented to use GET parameters like get_changeform_initial_data, but it wouldn't be trivial due to the list nature of formset initial data.

[Philip James]

Currently the PR has merge conflicts

[Tim Graham]

I think we should add a more general customization hook that allows customizing the parameters passed to the ​formset initialization (which includes initial data). That could also allow the use case of #27240 which requires adding form_kwargs': {'request': request} to formset_params.

[Manav Agarwal]

Considering the model and admin as defined below.
models.py

class Author(models.Model):
    name = models.CharField(max_length=100)


class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)

admin.py

class BookInline(admin.StackedInline):
    model = Book
class AuthorAdmin(admin.ModelAdmin):
    inlines = [
        BookInline,
    ]

admin.site.register(Author, AuthorAdmin)

Is it a good idea to pass the initial vales of bookinline fields by using get request in such a way like

http://127.0.0.1:8000/admin/polls/author/add/?name=Author_name&book_1_name=book1_name_value&book_2_name=book2_name_value

Example:

http://127.0.0.1:8000/admin/polls/author/add/?name=william_shakespeare&book_1_name=Hamlet&book_2_name=Romeo_and_Juliet

Please update me if the idea seems fine so that I may create a PR in order to solve the issue.

[Manav Agarwal]

​PR

[Mariusz Felisiak]

Per Nick's comments.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 3119a6d:

Fixed #26607 -- Allowed customizing formset kwargs with ModelAdmin.get_formset_kwargs().

Thanks Nick Pope for reviews.