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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def to_db(catchment, session, method='create', autocommit=False):
 """ Load catchment object into the database. A catchment/station number (:attr:`catchment.id`) must be provided. If :attr:`method` is set to `update`, any existing catchment in the database with the same catchment number will be updated. :param catchment: New catchment object to replace any existing catchment in the database :type catchment: :class:`.entities.Catchment` :param session: Database session to use, typically `floodestimation.db.Session()` :type session: :class:`sqlalchemy.orm.session.Session` :param method: - ``create``: only new catchments will be loaded, it must not already exist in the database. - ``update``: any existing catchment in the database will be updated. Otherwise it will be created. :type method: str :param autocommit: Whether to commit the database session immediately. Default: ``False``. :type autocommit: bool """


    if not catchment.id:
        raise ValueError("Catchment/station number (`catchment.id`) must be set.")
    if method == 'create':
        session.add(catchment)
    elif method == 'update':
        session.merge(catchment)
    else:
        raise ValueError("Method `{}` invalid. Use either `create` or `update`.")
    if autocommit:
        session.commit()