Question

I’m having some issues with double-posting on my site. I figure a simple unique constraint across all the relevant fields will solve the issue on a database level, but then it just produces a nasty error page for the user. Is there a way I can turn this into a pretty form error instead? Like a non_field_error? Or what approach should I take?

Answer 1

Maybe something like this will help you:

class YourForm(forms.Form):
    # Everything as before.
    ...

    def clean(self):
        cleaned_data = self.cleaned_data
        your_unique_key = cleaned_data['your_unique_key']

        if your_unique_key and YourModel.objects.get(your_unique_key=your_unique_key):
            raise forms.ValidationError("not unique")

        # Always return the full collection of cleaned data.
        return cleaned_data

The clean() method will allow you to access all fields of the form which might be useful if you have a combined unique key. Otherwise a (sightly shorter) clean_your_unique_key() might suit you better.

And please note that under rare circumstances (race conditions) the form validation might not report a duplicate entry (but it’s of course reported by the database engine). But for most applications the provided example will be the easier and more maintainable one, so I still recommend this approach.