Custom validation of Django inline formset

custom-validation-inlineformset.py

class ProductAutochangeInlineFormSet(
    forms.inlineformset_factory(ProductAutochangeSet, ProductAutochange, form=ProductAutochangeForm, extra=1)):
    def clean(self):
        super(ProductAutochangeInlineFormSet, self).clean()

        product_pks = []
        num_productautochanges = 0
        for form in self.forms:
            if not form.is_valid():
                continue
            if form.cleaned_data and not form.cleaned_data.get('DELETE'):
                num_productautochanges += 1
                if form.cleaned_data["product"].pk in product_pks:
                    form.add_error("product", "Product {} cannot be autochanged multiple times.".format(
                        form.cleaned_data["product"]))
                else:
                    product_pks.append(form.cleaned_data["product"].pk)

        if num_productautochanges < 1:
            raise ValidationError("At least one product autochange needs to be defined.")