Django unique constraint + form errors

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.

Answer 2

as far as a 'nasty error page' for the user, Django lets you customize your own 500,404 and probably other pages. general info on that:

In order to use the Http404 exception to its fullest, you should create a template that is displayed when a 404 error is raised. This template should be called 404.html and located in the top level of your template tree.

-- http://docs.djangoproject.com/en/dev/topics/http/views/

another nice way, not as DRY as tux21b's solution but perhaps a little easier to understand for a one-time solution, might be to catch the error intelligently. one way is to do so without even bothering to violate the constraint - a simple query should verify whether the user is about to do something illegal.

okToUpdate=MyModel.objects.filter(parameters=values...).count()  

if okToUpdate>0:  # an object already exists  
    errorExists=True  
    errors={customError:customMessage}  

...  

if errorExists:  
     return render_to_response(errors,'customErrorPage.html')  

else:  
    # return whatever you normally would return  

you then use render_to_response to render a custom error page.

(another way is to allow the database violation to occur, then catch that error and do the same thing... i theorize that a DB gets slightly less stress doing a lookup than handling an exception but it's up to you how you like to do things).

JB