return specific properties of array element - MongoDB/ Meteor

Question

I have documents in games collection.Each document is responsible for holding the data that requires to run the game. Here's my document structure

{
    _id: 'xxx',
    players: [
            user:{} // Meteor.users object
            hand:[] //array
            scores:[]
            calls:[]
        ],
    table:[],
    status: 'some string'


}

Basically this is a structure of my card game(call-bridge). Now what I want for the publication is that the player will have his hand data in his browser( minimongo ) along with other players user, scores, calls fields. So the subscription that goes down to the browser will be like this.

{
    _id: 'xxx',
    players: [
            {
                user:{} // Meteor.users object
                hand:[] //array
                scores:[]
                calls:[]
            },
            {
                user:{} // Meteor.users object
                scores:[]
                calls:[]
            },
            //  2 more player's data, similar to 2nd player's data

        ],
    table:[],
    status: 'some string'


}

players.user object has an _id property which differentiates the user. and in the meteor publish method, we have access to this.userId which returns the userId who is requesting the data.It means I want the nested hand array of that user whose _id matches with this.userId. I hope this explanations help you write more accurate solution.

erickeno · Accepted Answer · 2016-06-24 20:25:19Z

1

What you need to do is "normalize" your collection. Instead of having hand,scores, calls in the players field in the Games collection, what you can do is create a separate collection to hold that data and use the user _id as the "Key" then only reference the user _id in the players field. For example.

Create a GameStats collection(or whichever name you want)

    {
_id: '2wiowew',
userId: 1,
hand:[],
scores:[],
calls:[],
}

Then in the Games collection

{
    _id: 'xxx',
    players: [userId],
    table:[],
    status: 'some string'


}

So if you want to get the content of the current user requesting the data

GameStats.find({userId: this.userId}).hand

EDIT

They do encourage denormalization in certain situations, but in the code you posted above, array is not going to work. Here is an example from the mongoDB docs.

{
_id: ObjectId("5099803df3f4948bd2f98391"),
name: { first: "Alan", last: "Turing" },
birth: new Date('Jun 23, 1912'),
death: new Date('Jun 07, 1954'),
contribs: [ "Turing machine", "Turing test", "Turingery" ],
views : NumberLong(1250000)
}

edited Jun 24, 2016 at 20:25

answered Jun 24, 2016 at 14:14

erickeno

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1 Comment

Faysal Ahmed Over a year ago

thanks for your suggestion. As MongoDB encourages denormalization, i'm looking for answer than matches with my structure. :)

Praneeth Gudumasu · Accepted Answer · 2016-06-24 12:36:00Z

0

To get a specific property from an array element you may write something as in the below line db.games.aggregate([{$unwind:"$players"},{$project:{"players.scores":1}}]); this gives us only the id and scores fields

answered Jun 24, 2016 at 12:36

Praneeth Gudumasu

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1 Comment

Faysal Ahmed Over a year ago

i don't think aggregation would work for my case. i need to return a find cursor for meteor publish

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