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_m2m(self):
        super()._save_m2m()
        campaign = Campaign.objects.get(pk=self.cleaned_data['campaign'])
        for producttype in campaign.product_types.all():
            Product.objects.get_or_create(
                order=self.instance,
                product_type=producttype)