Mongoose - increment a field value in nested array

Question

I'm new to Mongoose and MongoDB and am trying to implement a scenario where I have to update a field value residing in a nested array.

For example, my DB entry is as follows:

{
    "_id" : ObjectId("581999ecf97860b77c1625f6"),      // this is userID
    "username" : "test",
    "__v" : 0,
    "polls" : [
            {
                    "url" : "xxxx",
                    "question" : "city or united",
                    "_id" : ObjectId("581999ec3ba2b15ad7fbcd50"),    //this is questionID
                    "option" : [
                            {
                                    "votes" : 0,
                                    "value" : "city",
                                    "_id" : ObjectId("581999ec3ba2b15ad7fbcd52")    //this is optionID
                            },
                            {
                                    "votes" : 0,
                                    "value" : "united",
                                    "_id" : ObjectId("581999ec3ba2b15ad7fbcd51")    //this is optionID
                            }
                    ]
            },
            {
                    "url" : "yyyy",
                    "question" : "chelsea or arsenal",
                    "_id" : ObjectId("58199d0ef11835685d3f41b7"),
                    "option" : [
                            {
                                    "votes" : 0,
                                    "value" : "chelsea",
                                    "_id" : ObjectId("58199d0ef11835685d3f41b9")
                            },
                            {
                                    "votes" : 0,
                                    "value" : "arsenal",
                                    "_id" : ObjectId("58199d0ef11835685d3f41b8")
                            }
                    ]
            }
    ]
}

I need to increment the votes of a particular option whenever it is selected from the front-end.

Below is what I've tried to implement so far but without success.

PollsSchema.methods.submitVote = function(id, callback){
var increment = { $inc: { 'polls.option.$.votes': 1 } };
var query = { 
    '_id': id.userID,
    'polls._id': id.questionID, 
    'polls.option._id': id.optionID
};

return this.model('Poll').update(query, increment, callback);
};

Below is my schema:

var PollsSchema = new Schema({
username: String,
polls: [
    {
        question: String,
        option: [
            {
                value: String,
                votes: Number
            }
        ],
        url: String
    }
]
});

Is there something that I'm doing incorrectly over here?

Thank you for your time and suggestions.

It's a known issue on MongoDB. there is already an open bug jira.mongodb.org/browse/SERVER-831 — molaga
– molaga, Commented Nov 2, 2016 at 13:20
Possible duplicate of Using positional operator with two-level hierarchies in Mongo — molaga
– molaga, Commented Nov 2, 2016 at 13:23

s7vr · Accepted Answer · 2016-11-06 16:14:50Z

1

You've the hit the Mongo db limitation where it doesn't support the nested array placeholder replacement. The positional operator can only be used for one level. So if you were to update the question like polls.$.question under polls, it will work fine. Also, mongo db doesnt have support for multiple placeholder something like polls.$.option.$.votes.

As pointed in the comment you can view that jira defect. Now coming to a workaround.

You can flatten your options so we can have it inside the polls. Something like this below. As long as you have unique id for question and option you should always can resolve to unique record.

{
    "_id": ObjectId("581999ecf97860b77c1625f6"), // this is userID
    "username": "test",
    "__v": 0,
    "polls": [{
        "url": "xxxx",
        "question": "city or united",
        "_id": ObjectId("581999ec3ba2b15ad7fbcd50"), //this is questionID
        "votes": 0,
        "value": "city",
        "option_id": ObjectId("581999ec3ba2b15ad7fbcd52") //this is optionID
    }, {
        "url": "xxxx",
        "question": "city or united",
        "_id": ObjectId("581999ec3ba2b15ad7fbcd50"), //this is questionID
        "votes": 0,
        "value": "united",
        "option_id": ObjectId("581999ec3ba2b15ad7fbcd51") //this is optionID
    }]
}

Now you can use something like this to increment the votes.

PollsSchema.methods.submitVote = function(id, callback) {
    var increment = {
        $inc: {
            'polls.$.votes': 1
        }
    };
    var query = {
        '_id': id.userID,
        'polls._id': id.questionID,
        'polls.option_id': id.optionID
    };

    return this.model('Poll').update(query, increment, callback);
};

Other option is to use aggregate pipeline and find the item and then update that returned object.

edited Nov 6, 2016 at 16:14

answered Nov 2, 2016 at 13:21

s7vr

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1 Comment

Yash Kapila Over a year ago

Thank you for response. I would give restructuring the schema a shot. But for the time being, I've used a workaround by which I'm finding the poll by questionID and then running a loop on the options and incrementing the votes if the option._id matches the optionID passed from frontend. I'm then using the save() to write it back to the DB.

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