def _cache_run_start(run_start, run_start_cache):
    """Cache a RunStart document

    Parameters
    ----------
    run_start : dict
        raw pymongo dictionary. This is expected to have
        an entry `_id` with the ObjectId used by mongo.

    run_start_cache : dict
        Dict[str, Document]

    Returns
    -------
    run_start : dict
        Document instance for this RunStart document.
        The ObjectId has been stripped.
    """
    run_start = dict(run_start)
    run_start.pop('beamline_config_id', None)
    oid = run_start.pop('_id', None)
    run_start_cache[run_start['uid']] = run_start
    run_start_cache[oid] = run_start
    return run_start
