Add new element to mongoDB document array only if a certain condition is met

Question

I have this mongoDB document structure:

Game = {
  _id: 'randomObjectId',
  players: [Array of players ObjectIds],
  maxPlayers: 2,
  status: 'created' (or 'fully-booked')
}

When a new player joins the game I want to add him to the players array and change the status of the game to fully-booked only if this condition is met: players.length < maxPlayers.

To recap:

Check inside the DB(using the query) if players.length < maxPlayers;
If yes update the document
If no send back an error response (optional)

Without this check it's easy to make the query using:

Game.findByIdAndUpdate('randomObjectId', {
  $addToSet: {
    players: 'playerObjId'
  },
  gameStatus: 'fully-booked'
}, {
  new: true
})

What I don't know is how to add this condition to this (or other type of) query. Any help is appreciated. Thanks

Your condition is broken, btw. If implemented as requested, it'd mark a game as "fully booked" when the game gets its first player (1 of 2). I suggest you separate marking as "fully booked" from adding players to the game. — Sergio Tulentsev
– Sergio Tulentsev, Commented Jun 3, 2021 at 15:10
And to your general question, "how do I add conditions to updates", I'd use findAndModify here. — Sergio Tulentsev
– Sergio Tulentsev, Commented Jun 3, 2021 at 15:35
Please check docs.mongodb.com/manual/reference/operator/aggregation/set/… — Alex Blex
– Alex Blex, Commented Jun 3, 2021 at 15:38
@AlexBlex: aggregation pipeline's $set is irrelevant here, though? Or am I missing something? — Sergio Tulentsev
– Sergio Tulentsev, Commented Jun 3, 2021 at 16:05
@SergioTulentsev the pipeline $set makes it possible to do conditional update with docs.mongodb.com/manual/reference/operator/aggregation/cond/… operator. — Alex Blex
– Alex Blex, Commented Jun 3, 2021 at 16:13

Dĵ ΝιΓΞΗΛψΚ · Accepted Answer · 2021-06-07 04:55:45Z

1

you can try an aggregation pipeline update like the following. however, if already fully booked, the command will return a null. also note that if you try to add a player id that already exists in the array, it won't be added.

db.collection.findOneAndUpdate(
    {
        _id: someObjectId,
        $expr: {
            $lt: [{ $size: "$players" }, "$maxPlayers"]
        }
    },
    [
        {
            $set: {
                players: { $setUnion: ["$players", [newPlayerObjectId]] }
            }
        },
        {
            $set: {
                status: {
                    $cond: {
                        if: { $lt: [{ $size: "$players" }, "$maxPlayers"] },
                        then: "created",
                        else: "fully-booked"
                    }
                }
            }
        }
    ],
    {
        returnNewDocument: true
    })

edited Jun 7, 2021 at 4:55

answered Jun 4, 2021 at 4:28

Dĵ ΝιΓΞΗΛψΚ

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2 Comments

Gabriel Lupu Over a year ago

$set throws an error but the $expr in the first part prevents me from adding an extra player, so it works in the end. Thanks

Dĵ ΝιΓΞΗΛψΚ Over a year ago

@GabrielLupu works for me without an issue. see here: streamable.com/linm5c that is in mongo shell though. i'm not sure if mongoose has support for pipeline updates.

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