def key2mongo(tup):
    """Return a string given an (int, string) plate-well key.

    Parameters
    ----------
    tup : (int, string) tuple
        A (plate, well) identifier.

    Returns
    -------
    mongo_id : string
        A string, suitable for representing a mongodb _id field.

    Examples
    --------
    >>> tup = (2490688, 'C04')
    >>> key2mongo(tup)
    '2490688-C04'
    """
    return str(tup[0]) + '-' + tup[1]