ModelAdmin form validation that cross-validates main form and inline formsets

[ldeluigi]

I'd like to validate the admin change form submission for my entity together with its inline formsets.
In particular, I need to compute and validate a field based on the model relationships, that is its inlines that were submitted.
I can't use any of the forms clean() method because they can't access other forms of the same submission, unless I'm parsing the request manually (duplicating the code from the inline formset arguments parser).

Proposal:
instead of calling the all_valid() method directly from _changeform_view() in the ModelAdmin implementation, you could first call an instance method like for example self.validate_formsets(form, formsets) which returns True if all the forms are valid.

The default implementation in ModelAdmin would be:

def validate_formsets(self, form, formsets):
    form_validated = form.is_valid()
    return all_valid(formsets) and form_validated

then users can override it to customize the behavior, including cross-validating forms cleaned data.

[Mariusz Felisiak]

Duplicate of #12780.