I've found very little about the functionality of matchData. What I'm trying to do is store a couple small pieces of data in matchData. From reading this Q&A post I've figured out much of what I need, but I'm stumped on this:

Here's my desired data:

"matchData": { "seats": [ { "player_id": "58a08a760a45b30567f848ab", "seat": 5 }, {"player_id": "blah", "seat": 3 ] , "dealer": "58a08a760a45b30567f848ab"}

 So two keys at the root level ("seats", which itself can contain many nodes within it, and "dealer", which is a single).

Through cloudcode I'm using the MatchFoundMessage to check the matchdata for "dealer", and if it doesn't exist, it adds it with a random participant of the match. That much works.

I have an Event-bound script that adds "seats" nodes to the MatchData. The first time it's called, it replaces everything in MatchData with the new seat entry. Subsequent seat entries get added to the "Seats" collection just fine. Borrowing heavily from the previous link, my Event script looks like this:

var seatNumberA = Spark.getData().seatNumber;
var matchIDA = Spark.getData().matchID;
var theMatch = Spark.getMultiplayer().loadMatch(matchIDA);
var seatNode = theMatch.getMatchData().seats;

var playerSending = Spark.getPlayer().getPlayerId();

if(seatNode === undefined)
{
    theMatch.setMatchData({"seats":[{"player_id": playerSending, "seat": seatNumberA}]});
}
else
{
    if(!contains(seatNode, playerSending))
    {
        seatNode.push({"player_id":playerSending, "seat":seatNumberA})
        theMatch.setMatchData({
            "seats" : seatNode
        })
    }
}

var playersToSendTo = Spark.getMultiplayer().loadMatch(matchIDA).getParticipants();

for (var i = 0; i<playersToSendTo.length;i++)
{
    Spark.message("playerSeat").setPlayerIds(playersToSendTo[i].getPlayer().getPlayerId()).setMessageData({"playerID":playerSending, "playerSeat":seatNumberA}).send();
};

function contains(a, obj) {
    for (var i = 0; i < a.length; i++) {
        if (a[i].player_id === obj) {
            return true;
        }
    }
    return false;
}

var baseNode = theMatch.getMatchData();