If the python function “is_valid” has an exception, how do I keep the input values that were entered on the django form?
I did search, maybe I don’t know a better way to search.
If there is a better way to use a Parent Child model, please tell me.

First I save the Parent model, then save the Child model in same view.

On exception, I want to rollback Parent and redirect user back to
the same page with the fields populated with their values.

I have combined 2 forms, each uses a different model.
I’m using class-based views.

The line below “_starting“, (print(forms), gives me:

{'parent_form': <Parent_2testForm bound=True, valid=True, fields=(title_test;modelname_id)>, 'child_form': <Child_testForm bound=True, valid=True, fields=(content_test)>}

urls.py
path('newpos/parent2',views.Parent2.as_view(),name='parent2'),

Models.py

class Parent_test(models.Model):
    title_test = models.CharField(max_length=50)
    modelname_id = models.IntegerField(null=False, default=0, unique=False)

class Child_test(models.Model):
    content_test = models.TextField()
    id_key = models.IntegerField(null=False, default=0, unique=False)
    parent_test = models.ForeignKey(Parent_test, on_delete=models.CASCADE)

forms.py

class Parent_2testForm(ModelForm):
    class Meta:
        model = Parent_test
        fields = ('title_test', 'modelname_id', )
        widgets = {
            'modelname_id': forms.TextInput(attrs={'type': 'hidden'}),
        }

class Child_testForm(ModelForm):
    class Meta:
        model = Child_test
        fields = ('content_test',)

Views.py

class Parent2(MultiModelFormView):
   form_classes = {
       'parent_form': Parent_2testForm,
       'child_form': Child_testForm,
   }
   def __init__(self, title_test="", content_test="", *args, **kwargs):
       self.title_test = title_test
       self.content_test = content_test

   template_name = 'newpos/parent2.html'
   def get_success_url(self):
       return reverse('new_purchase_order')

   def forms_valid(self, forms):
       print("__c_inside_Parent2_forms_valid")

       try:
          with transaction.atomic():
              print("_______starting______")
              print(forms)

              parentsave = forms['parent_form'].save()
              parentsave.modelname_id = parentsave.id

              Parent_test.objects.update_or_create(id=parentsave.id, defaults={'modelname_id': parentsave.id})

              child2 = forms['child_form'].save(commit=False)
              child2.parent_test_id = parentsave.id

              child2.id_key = parentsave.id

              child3 = child2
              child3.save()

       except ValueError as e:
            amsg = "__ValueERROR_exception_" + str(e)

            # raise # to remove record saved in Parent
            context = {}
            messages.error(self.request, amsg)
            return HttpResponseRedirect(self.request.path_info)

       except Exception as e:
          amsg = "__ERROR_exception_" + str(e)
          # raise  # to remove record saved in Parent
          messages.error(self.request, amsg)
          return HttpResponseRedirect(self.request.path_info )

       return super(Parent2, self).forms_valid(forms)

I want to send the input values back in the form.

I have tried “request” option, but that did not work.