Please help me with this: when I add new data, it will throw an error

SqlException: Violation of PRIMARY KEY constraint ‘PK__SaleOrde__C3905BAFABA6B181’. Cannot insert duplicate key in object ‘dbo.SaleOrder’. The duplicate key value is (0).

because the OrderID doesn’t get automatically set.

Here it’s my code – controller:

using DoAnLTHDT.Models;
using DoAnLTHDT.ViewModel;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Services.Description;

namespace DoAnLTHDT.Controllers
{
    public class MasterDetailController : Controller
    {
        private UserManageEntities objManageEntities;

        public MasterDetailController()
        {
            objManageEntities = new UserManageEntities();
        }

        public ActionResult Index()
        {
            return View();
        }

        [HttpPost]
        public JsonResult Index(SaleOrderViewModel objSaleOrderViewModel)
        {
            SaleOrder objSalesOrder = new SaleOrder();
            objSalesOrder.AddressCus = objSaleOrderViewModel.AddressCus;
            objSalesOrder.CustomerName = objSaleOrderViewModel.CustomerName;
            objSalesOrder.OrderNumber = objSaleOrderViewModel.OrderNumber;
            objManageEntities.SaleOrders.Add(objSalesOrder);
            objManageEntities.SaveChanges();

            objSalesOrder = 
                objManageEntities.SaleOrders.Single(model =>
                model.OrderNumber == objSaleOrderViewModel.OrderNumber
            );

            foreach (var item in objSaleOrderViewModel.ListOfSaleOrderDetail)
            {
                SaleOrderDetail objSaleOrderDetail = new SaleOrderDetail();
                objSaleOrderDetail.Discount = item.Discount;
                objSaleOrderDetail.ItemName = item.ItemName;
                objSaleOrderDetail.OrderID = objSalesOrder.OrderID;
                objSaleOrderDetail.Quantity = item.Quantity;
                objSaleOrderDetail.Total = item.SubTotal;
                objSaleOrderDetail.UnitPrice = item.UnitPrice;
                objManageEntities.SaleOrderDetails.Add(objSaleOrderDetail);
                objManageEntities.SaveChanges();
            }

            return Json(data: new {Success = true, Message ="Data Add"}, JsonRequestBehavior.AllowGet);
        }
    }
}

Script ajax and jQuery

<script type="text/javascript">
    $(document).ready(function () {
        $("#btnDelete").hide();
    });
    function AddSaleOrderRow() {
        $("#btnDelete").show();
        var tblBody = $("#tblSaleOrderDetails").find("tbody");
        var tblLastRow = tblBody.find("tr:last");
        var tblClone = tblLastRow.clone().find("input[type=text]").val('').end();
        tblLastRow.after(tblClone);
        $("#btnDelete").hide();
    };
    function DeleteSaleOrderRow(btnDelete) {
        $(btnDelete).closest("tr").remove();
        CalculateSubTotal();
    }
    function ResetCustomerDetails() {
        $("#txtCustomerName").val('');
        $("#txtAddessCus").val('');
        $("#txtOrderNumber").val('');
        $("#txtTotal").val('');
        var tblBody = $("#tblSaleOrderDetails").find("tbody");
        tblBody.find('tr:gt(0)').remove();

        tblBody.find("tr").each(function () {
            $(this).find("input[type=text]").val('');
        });
    }
    function CalculateSubTotal() {
        var Total = 0.0;
        var tblBody = $("#tblSaleOrderDetails").find("tbody");
        tblBody.find("tr").each(function () {
            var Quantity = $(this).find("td:eq(1) input[type=text]").val(); 
            var UnitPrice = $(this).find("td:eq(2) input[type=text]").val(); 
            var Discount = $(this).find("td:eq(3) input[type=text]").val();
            var SubTotal = (Quantity * UnitPrice) - Discount;
            Total = Total + SubTotal;
            $(this).find("td:eq(4) input[type=text]").val(SubTotal)
            $("#txtTotal").val(Total);
        });
    }
    function SaveOrderDetails() {
        ListOfSaleOrderDetail = [];
        var tblBody = $("#tblSaleOrderDetails").find("tbody");
        tblBody.find("tr").each(function () {
            var ItemName = $(this).find("td:eq(1) input[type=text]").val();
            var Quantity = $(this).find("td:eq(2) input[type=text]").val();
            var UnitPrice = $(this).find("td:eq(3) input[type=text]").val();
            var Discount = $(this).find("td:eq(4) input[type=text]").val();
            var SubTotal = (Quantity * UnitPrice) - Discount;
            ListOfSaleOrderDetail.push({
                "ItemName": ItemName,
                "Quantity": Quantity,
                "UnitPrice": UnitPrice,
                "Discount": Discount,
                "SubTotal": SubTotal,
            });

        });
        SaleOrderViewModel = {
            "OrderNumber": $("#txtOrderNumber").val(),
            "CustomerName": $("#txtCustomerName").val(),
            "AddressCus": $("#txtAddressCus").val(),
            "ListOfSaleOrderDetail": ListOfSaleOrderDetail
        };
        $.ajax({
            async: true,
            type: 'POST',
            dataType: 'JSON',
            url: '/MasterDetail/Index',
            data: JSON.stringify(SaleOrderViewModel),
            contentType: 'application/json; charset=utf-8',
            success: function (data) {
                if (data.success) {
                    alert(data.Message);
                    ResetCustomerDetails()
                }
                else {
                    alert(data.Message);
                }
            },
            error: function (data) {
                alert("loi");
            }
        });
    }
</script>