Pass url parameter to Django ModelForm without rendering it as input on form

Question

I have a url /<subject_id>/comments/new/ which renders a Django ModelForm. I am using a view class derived from FormView to process the form. I wish to do the following:

subject_id should not appear on the rendered form.
subject_id should be added to the form prior to is_valid() being called, or if this is not possible should be added to the Comment instance.

forms/comment_form.py:

class CommentForm(ModelForm):
    class Meta:
        model = Comment
        fields = ['text']

views.py:

class CommentCreate(FormView):
    form_class = CommentForm

    def form_valid(self, form):
        # Do some stuff to the validated Comment instance
        # Maybe save the comment, maybe not
        return super().form_valid(form)

How do I do this? If I add subject_id as a field in CommentForm then it appears on the rendered form. If I don't then the form is instantiated with subject_id present from `self.kwargs['subject_id'] and complains of an "unexpected keyword argument".

what is your model ?

Dimitris Kougioumtzis
– Dimitris Kougioumtzis

2018-06-17 12:12:42 +00:00
Commented Jun 17, 2018 at 12:12 — Dimitris Kougioumtzis
– Dimitris Kougioumtzis, Commented Jun 17, 2018 at 12:12

Tintin · Accepted Answer · 2018-06-22 09:43:11Z

2

After some hunting around in the docs I have discovered that the correct answer is to use the get_form() method to pre-populate the form with the data that I don't want to appear on the form, but that needs to be present for validation.

class CommentCreate(FormView):
    form_class = CommentForm

    def get_form(self):
        self.subject= get_object_or_404(Subject, id=self.kwargs['subject_id'])
        partial_comment = Comment(user=self.request.user, subject=self.subject)
        form = CommentForm(**self.get_form_kwargs(), instance=partial_comment)
        return form

answered Jun 22, 2018 at 9:43

Tintin

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1 Comment

James C Over a year ago

The line form = CommentForm(**self.get_form_kwargs(), instance=partial_comment) can cause problems by redeclaring instance. You may need to update **self.get_form_kwargs() instead of adding it to the instantiation of the form.

neverwalkaloner · Accepted Answer · 2018-06-17 06:05:09Z

1

You can remove subject_id from form fields:

class CommentForm(ModelForm):
    class Meta:
        model = Comment
        fields = ['text']

And add it to new comment object in form_valid method like this:

class OrderCreate(FormView):
    form_class = CommentForm

    def form_valid(self, form):
        subject_id = self.kwargs['subject_id']
        subject = Subject.objects.get(id=subject_id)  
        form.instance.subject_id = subject
        return super().form_valid(form)

answered Jun 17, 2018 at 6:05

neverwalkaloner

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4 Comments

Tintin Over a year ago

Unfortunately this does not work. I get the error: 'CommentForm' has no field named 'subject'.

neverwalkaloner Over a year ago

@Tintin this code shouldn't raise the error. Problem is somewhere else. Can you post template and full traceback?

Tintin Over a year ago

The error is being throw by the Comment.clean() which references Subject. I need to populate the form data with subject_id before is_valid() is called.

neverwalkaloner Over a year ago

@Tintin ok, can you post full traceback and template also?

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