forms.py

class OrderForm(forms.ModelForm):
    class Meta:
        model = Order
        fields = ['book_title', 'customer', 'internal_design', 'paid', 'start_date']

    campaign = forms.ModelMultipleChoiceField(queryset=Campaign.objects.all())

    def save(self, commit=True):
        campaign = Campaign.objects.get(pk=self.cleaned_data['campaign'])
        # I need to create a Product object for each ProductType in campaign.producttypes.
        # Product has a foreign key to Order. How do I create and associate these without
        for producttype in campaign.product_types:
            Product.objects.update_or_create(
                order=self.instance,
                product_type=producttype,
                defaults=???)
        return super().save(commit=commit)