MongoDb c# 2.0 driver - how can i calculate count of aggregations on server?

Question

When i ran dbCollection.Aggregate().Group(groupby).Match(query) I haven't any extention method to get a count of records what was aggregated. But i can run ToListAsync and then get Count(). How can i receive a count of records ? I have MongoDb v 3.0.2 and c# driver 2.0

Answer 1

You should add one more group with $sum

var result = await collection
        .Aggregate()
        .Group(groupby)
        .Match(query)
        .Group(new BsonDocument
        {
            { "_id", "_id" },
            {"count", new BsonDocument("$sum", 1)}
        })
        .FirstAsync();
        var count = result["count"].AsInt32;