public void EnsureCompoundIndex (List<KeyValuePair<string, bool>> fields,
            string collectionName, bool setUnique = true)
        {
            var collection = _database.GetCollection<BsonDocument>(collectionName);
            IndexKeysDefinition<BsonDocument> keys;
            CreateIndexOptions options = new CreateIndexOptions();


            foreach (KeyValuePair<string, bool> field in fields)
                if (field.Value == true)
                    keys.Add.Ascending (field.Key);
                else
                    keys.Add.Descending (field.Key);
            }

            // Set options
            if   (setUnique) options.Unique = true;
            else             options.Unique = false;

            // Create index
            collection.Indexes.CreateOne(keys, options);
        }