Code sample 1:

                    IMongoCollection<Liability> collection = this.GetDbCollection();

                    var result = await collection.Aggregate()
                                           .Match(l => items.Contains(l.CollectionId))
                                           .Group(
                                               new BsonDocument
                                                   {
                                                       { "_id", "$collectionId" },
                                                       { "totalAmount", new BsonDocument { { "$sum", "$totalAmount" } } },
                                                       { "ddTotalAmount", new BsonDocument { { "$sum", "$details.ddTotalAmount" } } },
                                                       { "checkTotalAmount", new BsonDocument { { "$sum", "$details.checkTotalAmount" } } },
                                                       { "totalItemsCount", new BsonDocument { { "$sum", "$totalItemsCount" } } },
                                                       { "count", new BsonDocument { { "$sum", 1 } } }
                                                   })
                                           .ToListAsync();

                    return result.Select(tot => BsonSerializer.Deserialize<LiabilitySetTotals>(tot.ToBsonDocument()));
					
Code sample 2:

                    IMongoCollection<Liability> collection = this.GetDbCollection();

                    var result = await collection.Aggregate()
                                           .Match(l => items.Contains(l.CollectionId))
                                           .Group(
                                               l => l.CollectionId,
                                               g => new LiabilitySetTotals()
                                                        {
                                                            Id = g.Key,
                                                            Count = g.Count(),
                                                            TotalAmount = g.Sum(l => l.TotalAmount),
                                                            TotalItemsCount = g.Sum(l => l.TotalItemsCount),
                                                            DdTotalAmount = g.Sum(l => l.Details.DdTotalAmount),
                                                            CheckTotalAmount = g.Sum(l => l.Details.CheckTotalAmount)
                                                        })
                                           .ToListAsync();

                    return result; 
					
---

    [BsonIgnoreExtraElements]
    [Serializable]
    [DataContract]
    public class LiabilitySetTotals
    {
        /// <summary>
        /// Gets or sets the Collection ID.
        /// </summary>
        [BsonId]
        [DataMember]
        public Guid Id { get; set; }

        /// <summary>
        /// Gets or sets the total amount.
        /// </summary>
        [BsonElement("totalAmount")]
        [BsonRepresentation(BsonType.Double, AllowTruncation = true)]
        [DataMember]
        public decimal TotalAmount { get; set; }

        /// <summary>
        /// Gets or sets the total amount.
        /// </summary>
        [BsonElement("ddTotalAmount")]
        [BsonRepresentation(BsonType.Double, AllowTruncation = true)]
        [DataMember]
        public decimal DdTotalAmount { get; set; }

        /// <summary>
        /// Gets or sets the total amount.
        /// </summary>
        [BsonElement("checkTotalAmount")]
        [BsonRepresentation(BsonType.Double, AllowTruncation = true)]
        [DataMember]
        public decimal CheckTotalAmount { get; set; }

        /// <summary>
        /// Gets or sets the total pay items count.
        /// </summary>
        [BsonElement("totalItemsCount")]
        [DataMember]
        public int TotalItemsCount { get; set; }

        /// <summary>
        /// Gets or sets the liabilities count.
        /// </summary>
        [BsonElement("count")]
        [DataMember]
        public int Count { get; set; }

        public LiabilitySetTotals()
        {
            this.Id = Guid.Empty;
        }
    }