[FIXED] How to handle UniqueConstraint failure in Django Meta Class?

Issue

I have created a UniqueConstraint Meta Class for my model, and it works when I test it by intentionally creating a duplicate but I am getting an error page instead of being redirected back to the original page. Where do I insert the code to go back to the page that I was just on?

I assume the issue is in the view, but I have no idea what code to put in there and the Django docs contain nothing alluding to this problem.

Model:

class StudentScheduledClass(models.Model):
    student = models.ForeignKey("users.User", on_delete=models.CASCADE, db_column="Student")
    scheduled_class = models.ForeignKey("ScheduledClass", on_delete=models.CASCADE, db_column="ScheduledClass")
    grade = models.FloatField(db_column="Grade", blank=True, null=True)

    class Meta:
        managed = True
        db_table = "StudentScheduledClass"
        verbose_name_plural = "StudentsScheduledClasses"
        constraints = [
            models.UniqueConstraint(fields=['student', 'scheduled_class'], name='student scheduled class restraint')
        ]

View:

class StudentScheduledClassCreateView(LoginRequiredMixin, CreateView):
    model = StudentScheduledClass
    context_object_name = "student_scheduled_class"
    fields = ["student"]

    def form_valid(self, form):
        scheduled_class = self.kwargs["scheduled_class"]
        form.instance.scheduled_class = ScheduledClass(scheduled_class)
        return super().form_valid(form)

    def get_success_url(self):
        scheduled_class = self.kwargs["scheduled_class"]
        return reverse("scheduled-class-detail", args={scheduled_class})

I'd like to just go back to the original page with an error message, instead I get this integrity error:

IntegrityError at /classes/student_scheduled_class_create/1/
UNIQUE constraint failed: StudentScheduledClass.Student, StudentScheduledClass.ScheduledClass
Request Method: POST
Request URL:    http://localhost:8000/classes/student_scheduled_class_create/1/
Django Version: 2.2.2
Exception Type: IntegrityError
Exception Value:    
UNIQUE constraint failed: StudentScheduledClass.Student, StudentScheduledClass.ScheduledClass

Solution

Your form only requires the field student so it's valid because you haven't given its instance a value for scheduled_class when it gets validated.

You should initialise your form with an instance for which scheduled_class is already set. You can do that in get_form_kwargs() or, since that method passes self.object as instance, you can do that even simpler in the post():

def post(self, request, *args, **kwargs):
    self.object = StudentScheduledClass(scheduled_class=kwargs['scheduled_class'])
    return super().post(request, *args, **kwargs)

Alternatively, although a bit less clean (I like to think that once form_valid() is called everything should be ok):

def form_valid(self, form):
    scheduled_class = self.kwargs["scheduled_class"]
    form.instance.scheduled_class = ScheduledClass(scheduled_class)
    try:
        return super().form_valid(form)
    except IntegrityError:
        return self.form_invalid(form)

The problem with the last one is that the form won't have any errors to show, so you'd probably want to add a Django message to the self.request before returning (assuming you display any message in your template).

Answered By - dirkgroten