def prev_next_group(project, group):
    groups = sorted(x for x in project.groups if x.submissions)
    try:
        index = groups.index(group)
    except ValueError:
        return None, None
    prev_group = groups[index - 1] if index > 0 else None
    next_group = groups[index + 1] if index + 1 < len(groups) else None
    return prev_group, next_group