def METHOD_NAME(self, bundle, request=None):
    errors = {}
    a = Vocabulary.objects.filter(
        display_name=bundle.data['display_name'],
        course=bundle.request.course)
    if len(a) > 0:  # vocabulary exists with this name
        if not bundle.obj or not bundle.obj.pk or a[0].pk != bundle.obj.pk:
            # a vocabulary already exists with this name
            msg = 'A %s concept exists. Please choose another name' \
                % bundle.data['display_name']
            errors['error_message'] = [msg]
    return errors