Count an array inside an array and then sum them MongoDB NodeJS

Question

I have an object that has an array of page objects and each page object has an array of questions.

Ex object:

{
Id: 1,
UserId: 14,
Deleted: false,
Collaborators: [],
Title: "Awesome",
Pages: [{
    Id: 1,
    Title: 'Jank',
    Questions: [
        { Id: 1, Content: 'Ask me about it' },
        { Id: 2, Content: 'Ask me about it again' }
    ]
}, {
    Id: 2,
    Title: 'Janker',
    Questions: [
        { Id: 1, Content: 'Tell me about it' },
        { Id: 2, Content: 'Tell me about it again' }
    ]
}]
}

What I am trying to do is to get a count of all the questions for the entire bas object. I am not sure how to do that. I have tried to use aggregate and $sum the total questions and then do another function to $sum those all together to get a total for the entire object. Unfortunately my $sum is not working like I thought it would.

Ex code (nodejs):

var getQuestionCount = function(id) {
    var cursor = mongo.collection('surveys').aggregate([{
            $match: {
                $or: [{
                    "UserId": id
                }, {
                    "Collaborators": {
                        $in: [id]
                    }
                }]
            }
        }, {
            $match: {
                "Deleted": false
            }
        }, {
            $unwind: "$Pages"
        },
        { $group: { _id: null, number: { $sum: "$Pages.Questions" } } }
    ], function(err, result) {
        //This log just gives me [object Object], [object Object]
        console.log('q count ' + result);
    });
}

Any idea how to do this? My end result from the example object above would ideally return 4 as the question count for the whole object.

Saleem · Accepted Answer · 2016-06-28 09:05:53Z

1

I'd try following shell query.

db.collection.aggregate([
  // filter out unwanted documents.
  {$match:{Id: 1}},

  // Unwind Pages collection to access Questions array
  {$unwind:"$Pages"},

  // Count items in Questions array
  {$project:{count: {$size:"$Pages.Questions"}}},

  // Finally sum items previously counted.
  {$group:{_id:"$_id", total: {$sum: "$count"}}}
])

Based on your sample document, it should return correct count of Questions.

{ 
    "_id" : ObjectId("57723bb8c10c41c41ff4897c"), 
    "total" : NumberInt(4)
}

answered Jun 28, 2016 at 9:05

Saleem

9,0582 gold badges22 silver badges34 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Ohjay44 Over a year ago

Thank you that worked great. The result is an array thought not a single object so i just did result[0].total

Collectives™ on Stack Overflow

Count an array inside an array and then sum them MongoDB NodeJS

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related