Mongo University answer to an exercise

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
 
namespace homework2
{
    class Program
    {
        static void Main(string[] args)
        {
           MainAsync(args).Wait();
            Console.WriteLine("press enter");
            Console.ReadLine();
        }
 
        private static async Task MainAsync(string[] args)
        {
            var connectionstring = "mongodb://127.0.0.1:27018";
            var client = new MongoClient(connectionstring);
            var db = client.GetDatabase("mongowork");
            var col = db.GetCollection("students");
            var group = new BsonDocument 
                { 
                    { "$group", 
                        new BsonDocument 
                            { 
                                { "_id", new BsonDocument 
                                             { 
                                                 { 
                                                     "student_id","$student_id"
                                                 } 
                                             } 
                                }, 
                                { 
                                    "minQuantity", new BsonDocument 
                                                 { 
                                                     { 
                                                         "$push", "$$ROOT"
                                                     } 
                                                 } 
                                } 
                            } 
                  } 
                };
            var x = await col.Aggregate()
                .Match(y => y.type == "homework")
                .Group(new BsonDocument{{"_id", "$student_id"}, {"minQuantity", new BsonDocument("$min", "$score")}})//, {"test", new BsonDocument("$push","$$ROOT")}})
                .SortBy(y => y.score)
                .ToListAsync();
 
 
           /* var filter = Builders.Filter.Eq("_id", "$student_id","minQuantity",  new BsonDocument("$min", "score")});*/
            /*var pipeline = new[] { group };
            var result = await col.Find(group).ToListAsync();*/
            foreach (var t in x)
            {
                await col.DeleteManyAsync(doc => doc.score.Equals(t.minQuantity));
                Console.WriteLine(t.student_id + t._id.ToString());
            }
        }
 
        public class students
        {
 
            [BsonElement]
            public object minQuantity
            {
                get;
                set;
 
            }
            [BsonElement]
            public object test
            {
                get;
                set;
 
            }
            [BsonElement]
            public int _id
            {
                get; set;
                
            }
 
            [BsonElement]
            public int student_id
            {
                get;
                set;
 
            }
 
            [BsonElement]
            public string type
            {
                get;
                set;
 
            }
 
            [BsonElement]
            public double score
            {
                get;
                set;
 
            }
 
            
 
        }
    }
}