I have a runtimeCollection with all of the players UserIds, a bool that is set when the player is in challenger mode, and other data. I'd like to send a message to all players that are currently in challenger mode. I create a list of all runtimeCollection data for the players that are in challenger mode, but I am not sure how to save just the playerIds from this list in to another list that can be passed in to the Spark.Message().setPlayerIds();

var playerDataList = Spark.runtimeCollection("playerData"); // this will get the collection of player data
var challengerStatus = Spark.getData().challengerStatus;
var playerID = Spark.getPlayer().getPlayerId(); // first we get the id of the current player


var challengerPlayersData = playerDataList.find({
	"challengerStatus": 1
}); // search the collection data for the entry with the same id as the player
if(challengerPlayersData.count() === null){
}else{
    var challengerPlayersId = [];
    for(var i; i < challengerPlayersData.count(); i++){
        challengerPlayersId[i] = [challengerPlayersData[i].playerID];
    }
    
    var currentChallenger = playerDataList.findOne({
	    "playerID": Spark.getPlayer().getPlayerId()
    }); // search the collection data for the entry with the same id as the player

    var messageForm = {};
        messageForm.playerID = currentChallenger.playerID;
        messageForm.playerName = currentChallenger.playerName;
        messageForm.playerHealth = currentChallenger.playerHealth;
        messageForm.playerMana = currentChallenger.playerMana;
        messageForm.playerChar = currentChallenger.playerChar;
    
    Spark.message("ChallengerStatus").setMessageData(messageForm)
    .setPlayerIds(challengerPlayersId).send();
}
// var messageForm = {};
// Spark.message("ChallengerStatus").setMessageData(messageForm).setPlayerIds(Spark.getPlayer().getPlayerId()).send();


var currentPlayer = {
	"challengerStatus": challengerStatus
}; // we construct a new player from the data we are about to input into the player data
playerDataList.update({
	"playerID": playerID
}, //Looks for a doc with the id of the current player
{
	"$set": currentPlayer
}, // Uses the $set mongo modifier to set old player data to the current player data
true, // Create the document if it does not exist (upsert)
false // This query will only affect a single object (multi)
);

 Thanks for any help!

Fantastic! thank you Liam!

Is there documentation that explains this API that I can study? I have not been able to find it.

My only other question within this script example is saving entire GSData variables to my message and retrieving them in Unity. 

Where  have:

var messageForm = {};
        messageForm.playerID = currentChallenger.playerID;
        messageForm.playerName = currentChallenger.playerName;
        messageForm.playerHealth = currentChallenger.playerHealth;
        messageForm.playerMana = currentChallenger.playerMana;
        messageForm.playerChar = currentChallenger.playerChar;
     
    Spark.message("ChallengerStatus").setMessageData(messageForm)
    .setPlayerIds(challengerPlayersId).send();

I'd like to be able to save just the currentChallenger runtimeCollectionData instead of each variable individually. Or, ideally I'd like to save the entire challenge object to the message so I can access it similar to message.challenge. from challenge messages (like ChallengeAcceptedMessage) in Unity. 

Here is a link to the topic I started on the subject. May be best to respond there if your so inclined.

Thanks again Liam, huge help!