def METHOD_NAME(db_ns, db_id):
    """Return an ID ensuring a namespace prefix if known to be needed."""
    prefix = get_bioregistry_prefix(db_ns)
    if not prefix:
        return db_id
    banana = registry[prefix].get('banana')
    banana_peel = registry[prefix].get('banana_peel', ':')
    if banana and not db_id.startswith(f'{banana}{banana_peel}'):
        return f'{banana}{banana_peel}{db_id}'
    return db_id