Offer grading choices from the assignment definition as potential form
            field values for 'grading'.
            When no object is given in the form, the this is a new manual submission

def formfield_for_dbfield(self, db_field, **kwargs):
        ''' Offer grading choices from the assignment definition as potential form
            field values for 'grading'.
            When no object is given in the form, the this is a new manual submission
        '''
        if db_field.name == "grading":
            submurl = kwargs['request'].path
            try:
                # Does not work on new submission action by admin or with a change of URLs. The former is expectable.
                submid = [int(s) for s in submurl.split('/') if s.isdigit()][0]
                kwargs["queryset"] = Submission.objects.get(
                    pk=submid).assignment.gradingScheme.gradings
            except:
                kwargs["queryset"] = Grading.objects.none()
        return super(SubmissionAdmin, self).formfield_for_dbfield(db_field, **kwargs)