[1.5.x] Fixed #21539 -- Added example of modelformset_factory's form argument

   >>> AuthorFormSet(queryset=Author.objects.none())

Changing the ``form``

---------------------

By default, when you use ``modelformset_factory``, a model form will

be created using :func:`~django.forms.models.modelform_factory`.

Often, it can be useful to specify a custom model form. For example,

you can create a custom model form that has custom validation::

    class AuthorForm(forms.ModelForm):

        class Meta:

            model = Author

            fields = ('name', 'title')

        def clean_name(self):

            # custom validation for the name field

            ...

Then, pass your model form to the factory function::

    AuthorFormSet = modelformset_factory(Author, form=AuthorForm)

It is not always necessary to define a custom model form. The

``modelformset_factory`` function has several arguments which are

passed through to ``modelform_factory``, which are described below.

Controlling which fields are used with ``fields`` and ``exclude``

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