Hur man får Mongo-data med aggregerad MongoDB version 3.0

Prova följande codeRefer - https://docs.mongodb.com/manual /referens/operatör/aggregation/eq/

https://docs.mongodb.com/manual/reference/operator/ aggregation/cond/

https://docs.mongodb.com/manual/reference/operator/ aggregering/grupp/

DB.aggregate([
                {$project:
                {
                    tag_id: '$tag_id',
                    unreadcount: { $cond: [ { $eq: [ '$unread', 'true' ] }, 1, 0 ] }
                }},
                { $group: {
                    _id: '$tag_id',
                    unread: { $sum: '$unreadcount'},
                }}
            ], function (err, results) {
                console.log(results);
            })