def get_validation_relationship(how: str) -> str:
    """
    Get the validation relationship for the parameter :param:`how` of the
    :ref:`safe_merge` function. This function is only used,
    if :param:`allow_inflation` is set to False.

    Parameters
    ----------
    how : str {‘left’, ‘right’, ‘outer’, ‘inner’, ‘cross’}
        Value with merge strategy to be converted.

    Returns
    -------
    str
        The validation strategy of the parameter :param:`how`

    Raises
    ------
    ValueError
        If :param:`how` is not a valid merge strategy.
    """
    if how == "left":
        return "many_to_one"
    elif how == "right":
        return "one_to_many"
    elif how == "inner":
        return "one_to_one"
    raise ValueError(f'Unsupported "how" argument {how}')