Generic Create View, ForeignKey, and Clean()

Using Django Forms & APIs
1

I am having a problem with trying to use CreateView, and then calling methods on the model, because the instance attribute is None until too late in the form process. Suppose this setup:

urls.py:

urlpatterns = [ path('<int:pk>/create/', ChildCreate.as_view()) ]

models.py:

class Parent(models.Model):
    pass

class Child(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.CASCADE)

    def clean(self):
        if self.parent.pk == 1:
            raise ValidationError("Parent 1 can't have children, because reasons")

views.py:

class ChildCreate(CreateView)
    model = child
    fields = []

The problem is that “parent” is not set anywhere in this workflow. As a result when the form is ready to save() it throws a “Parent is None” error.

I can fix this by overriding form_valid() in views.py:

class ChildCreate(CreateView)
    model = child
    fields = []

    def form_valid(self, form):
        # copy url PK to the form so it has the value attached
        form.instance.parent_id = self.kwargs["pk"]

        return super().form_valid(form);

…but this does not fix the problem with the clean() method in models.py, because that is getting called before form_valid.

Where is the proper place to set the parent ID for the new child, so it’s available for clean and save methods?

2

Hi Greg.

Assuming you know the parent early, I’d probably override get_form() to set it when the form is first created.

3

I’m pretty certain that won’t work, as instance isn’t created until _post_clean():

5

Right when the form is instantiated, it creates the instance if one is not provided:

        # in BaseModelForm.__init__()
        if instance is None:
            # if we didn't get an instance, instantiate a new one
            self.instance = opts.model()

So then you can set values on the instance, and as long as they’re not overridden by your form data in construct_instance() they’ll be available.

So something like:

def get_form(...):
     form = super().get_form(...)
     form.instance.parent = parent_obj   # Generally try to not use the `_id` attributes. 
     return form

If you can get_form_kwargs() is perhaps nicer, but sometimes these views have too many methods to remember them all. :slightly_smiling_face:

I hope that makes sense.

Kind Regards,

Carlton

1 Like
6

Passing the instance of the parent like this should work.

class ChildCreate(CreateView):
      model = Child
      fields = '__all__'
  
      def form_valid(self, form):
          form.instance.parent = Parent.objects.get(id=self.kwargs.get('pk'))
          return super(ChildCreate).form_valid(form);