/// <summary>
        /// Ensures that a collections have a compound index of two fields.
        /// </summary>
        /// <param name="fieldName1">Field to be used as first part of the index.</param>
        /// <param name="fieldName2">Field to be used as second part of the index.</param>
        /// <param name="collectionName">Collection to apply this index.</param>
        /// <param name="indexDirection">Index direction. 1 means ascending,
        /// 0 means descending. Each bit corresponds to a field, by order.</param>
        /// <param name="setUnique">If set to <c>true</c> set unique.</param>
        public void EnsureCompoundIndex (string fieldName1, string fieldName2,
            string collectionName, byte indexDirection=11, bool setUnique = true)
        {
            var collection = _database.GetCollection<BsonDocument>(collectionName);
            IndexKeysDefinition<BsonDocument> keys;
            CreateIndexOptions options = new CreateIndexOptions();

            // Set keys
            switch (indexDirection)
            {
                case 00:
                    keys = Builders<BsonDocument>.IndexKeys
                    .Descending (fieldName1)
                    .Descending (fieldName2);
                    break;
                case 01:
                    keys = Builders<BsonDocument>.IndexKeys
                    .Descending (fieldName1)
                    .Ascending  (fieldName2);
                    break;
                case 10:
                    keys = Builders<BsonDocument>.IndexKeys
                    .Ascending  (fieldName1)
                    .Descending (fieldName2);
                    break;
                case 11:
                    keys = Builders<BsonDocument>.IndexKeys
                    .Ascending (fieldName1)
                    .Ascending (fieldName2);
                    break;
                default:
                    throw new Exception ("indexDirection must have a value " +
                        "between 00 and 11 for this overload.");
            }

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

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