private Loan(long bookId, long userId, DateTimePeriod dateTimePeriod)
 {
     _bookId         = bookId;
     _userId         = userId;
     _dateTimePeriod = dateTimePeriod;
     _active         = true;
 }
        public static Loan Create(long bookId, long userId, DateTimePeriod dateTimePeriod)
        {
            var loan = new Loan(bookId, userId, dateTimePeriod);

            return(loan);
        }