Linq - Sequence contains more than one element

Question

i am creating an invoice with multiple invoice items. this is how the invoice looks with multiple invoice items:

when i do a refresh of the whole page, it breaks and throws this error:

 Sequence contains more than one element

this is my code:

   public ActionResult Create(long userId, long invoiceId)
    {
        ViewBag.InvoiceId = invoiceId;
        ViewBag.UserId = userId;
        ViewBag.UserId = userId;
        var objInvoiceItems = (from i in db.tblinvoiceitems
                      where i.InvoiceId == invoiceId
                      select i
                 ).ToList();

        var a = (from o in objInvoiceItems
                 select new InvoiceItemViewModel {
                     Description = o.DESCRIPTION,
                      Quantity = o.Quantity,
                       Rate = o.Rate,
                       Id = o.ID,
                       InvoiceId = o.InvoiceId
                 }).ToList();

        var objInvoice = (from i in db.tblinvoices
                          where i.ID == invoiceId
                          select i
                 ).ToList();

        var obj = (from i in objInvoice
                   join j in a
                   on i.ID equals j.InvoiceId
                   select new InvoiceCreateViewModel
                   {
                       AmountPaid = i.AmountPaid,
                       DueDate = i.DueDate,
                       InvoiceNo = i.ID,
                       UserId = i.UserId,
                       InvoiceItems = a
                       //InvoiceItems = j
                   }).SingleOrDefault();
        //obj.InvoiceItems = ListInvoiceItems(invoiceId.ToString()).Model;
        if (obj == null)
        {
            obj = new InvoiceCreateViewModel();
        }
        return View(obj);
    }


   public class InvoiceCreateViewModel
{
    public long? UserId { get; set; }
    public double? AmountPaid { get; set; }
    public DateTime? DueDate { get; set; }

    public double? TotalAmount { get; set; }
    public long InvoiceNo { get; set; }
    public double? Rate { get; set; }
    public string Description { get; set; }
    public int? Quantity { get; set; }

    public IEnumerable<InvoiceItemViewModel> InvoiceItems { get; set; }

}

It's breaking at the line SingleOrDefault(). Invoice is always one and InvoiceItems can be multiple. I am handling Add new Invoice Items in a different function through jquery so this function only gets hit once when creating a new invoice with no invoice items.

I want to return one invoice with multiple invoice items;

Answer 1

You can probably do all your Linq queries at once (less overhead)

   var obj = (from invoice in db.tblinvoices
              where invoice.ID == invoiceId
              join items in db.tblinvoiceitems
              on invoice.ID == items.InvoiceId
              select new InvoiceCreateViewModel
                   {
                       AmountPaid = invoice.AmountPaid,
                       DueDate = invoice.DueDate,
                       InvoiceNo = invoice.ID,
                       UserId = invoice.UserId,
                       InvoiceItems = new List<InvoiceItemViewModel> {
                           Description = items.DESCRIPTION,
                           Quantity = items.Quantity,
                           Rate = items.Rate,
                           Id = items.ID,
                           InvoiceId = items.InvoiceId
                       }
                   }
              ).SingleOrDefault();

Answer 2

If you want to combine classes into a single that model that can be called using the Razor engine declare a viewModel at the top of your view like so

@model ViewModelExample

Simply create a new model that will have an invoice and a list of invoice items

public class ViewModelExample
{
   public Invoice invoice{get; set;}
   public List<InvoiceItems> itemsList{get; set;}
}

Assign to the datatype ViewModelExample the correct objects like so

modelExample.invoice = obj
modelExample.itemsList= objInvoice

Afterwards return the modelExample object and use Razer to display the result for each of the properties provided in the ViewModelExample class