A simple C# script to demo MongoDB aggregation performing group by count.

groupby_count_aggregation.cs

using System;
using MongoDB.Bson;
using MongoDB.Driver;
using System.Threading.Tasks;
using System.Linq;

/* Where document structure stored in localhost MongoDB : {token:"word"} 
*/

namespace Aggregation_01
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Start Async ");
            var client = new MongoClient("mongodb://localhost:27017");
            var database = client.GetDatabase("test");
            var collection = database.GetCollection<BsonDocument>("tokens");

            Task t = queryDatabase(collection);
            t.ContinueWith((str) =>
            {
                Console.WriteLine(str.Status.ToString());
                Console.WriteLine("Query Ends.");
            });
            t.Wait();
            Console.ReadKey();
        }

        public async static Task<string> queryDatabase(IMongoCollection<BsonDocument> collection)
        {
            Console.WriteLine("Query Starts...");

            var aggregate = collection.Aggregate()
                                       .Group(new BsonDocument { { "_id", "$token" }, { "count", new BsonDocument("$sum", 1) } })
                                       .Sort(new BsonDocument { { "count", -1 } })
                                       .Limit(10);
            var results = await aggregate.ToListAsync();
            foreach (var obj in results)
            {
                Console.WriteLine(obj.ToString());
            }
            return "finished";
        }
    }
}

Hi is it possible to use Expressions in Group() instead of defining a BsonDocument?

I have quite the same or similar question as @tanvi-khade.
I'm looking for some examples of Group() but instead of using BsonDocument I would like to see how it could be done with Lambdas for example:(not real example)
var aggregate = collection.Aggregate() .Group(c =>c.Date) ...
Again, this is not at all a real example. In fact, is what I want to find: a good example that I can follow.

Hi @tanvi-khade and @sebainones,

Assuming that you have the class mapping, then you should be able to. Extending the example above, say you have these two class mappings:

        public class MyClass 
        {
            [BsonId]
            private ObjectId Id {get; set;}
            [BsonElement("token")]
            public string Token { get; set; }
        }

        internal class Output
        {
            public ObjectId Id { get; set; }    
            public string Token {get; set;}
            public double Count { get; set; }
        }

Then, in your aggregation pipeline you could use the following to return a similar result:

            var docs = collection.Aggregate()
                                    .Group(y=>y.Token,
                                           z => new { 
                                               Token = z.Key, 
                                               Count = z.Sum(a => 1)
                                            }
                                    ).ToList();

If you have further questions, please post a question on https://developer.mongodb.com/community/forums/ with the following information:

• MongoDB .NET/C# driver version
• Example code snippets
• Example document input (in the database)
• Expected output

Regards,
Wan.

Hi @sindbach how to use this group pipeline with in Generic Types ? Is it possible ?