Add instance to database.

    :param model: a string, model name in rio.models
    :param _commit: control whether commit data to database or not. Default True.
    :param \*\*kwargs: persisted data.
    :return: instance id.

def add_instance(model, _commit=True, **kwargs):
    """Add instance to database.

    :param model: a string, model name in rio.models
    :param _commit: control whether commit data to database or not. Default True.
    :param \*\*kwargs: persisted data.
    :return: instance id.
    """
    try:
        model = get_model(model)
    except ImportError:
        return None

    instance = model(**kwargs)
    db.session.add(instance)

    try:
        if _commit:
            db.session.commit()
        else:
            db.session.flush()
        return instance.id
    except IntegrityError:
        db.session.rollback()
        return