public void ReturnBook(BookIssuance issuance)
 {
     if(!issuance.RealReturnDate.HasValue)
     {
         issuance.RealReturnDate = DateTime.Now;
         _context.SaveChanges();
     }
 }
 public bool GiveBook(int userId, int bookId, DateTime requredReturnDate)
 {
     var newIssuance = new BookIssuance
     {
         UserId = userId,
         BookId = bookId,
         RequiredReturnDate = requredReturnDate,
         IssuanceDate = DateTime.Now
     };
     if (!_context.Users.ToList().Single(u => u.UserId == userId).IsBanned)
     {
         _context.BookIssuances.Add(newIssuance);
         _context.SaveChanges();
         return true;
     }
     return false;
 }