Fetch Data query from MVC controller is not showing 2 columns

We can use eager loading with the Include method to load related entities efficiently. You are already doing this, which is good. Instead of calling ToList() on the entire dataList object, you can use AsEnumerable() to load only the necessary data into memory. This can improve performance if the original query returns a large amount of data. Instead of using anonymous types in the select statement, you can create a custom model class to hold the selected properties. This can make your code more readable and maintainable.

public class SalesDetailModel
{
    public int SalesDetailId { get; set; }
    public int Quantity { get; set; }
    public int ProductId { get; set; }
    public string ProductName { get; set; }
    public int SalesId { get; set; }
    public DateTime OrderDate { get; set; }
    public decimal SubTotal { get; set; }
    public decimal DiscountPercent { get; set; }
    public decimal VatPercent { get; set; }
    public decimal TotalAmount { get; set; }
}

public ActionResult getNewOrder()
{
    using (var db = new ASPNETMASTERPOSTEntities())
    {
        var dataList = db.tblSales.Include("tblSalesDetails");
        var modifiedData = dataList.AsEnumerable().SelectMany(x => x.tblSalesDetails.Select(d => new SalesDetailModel
        {
            SalesDetailId = d.SalesDetailId,
            Quantity = d.Quantity,
            ProductId = d.ProductId,
            ProductName = d.tblProduct.ProductName,
            SalesId = x.SalesId,
            OrderDate = x.OrderDate,
            SubTotal = x.SubTotal,
            DiscountPercent = x.DiscountPercent,
            VatPercent = x.VatPercent,
            TotalAmount = x.TotalAmount
        }));
        return Json(modifiedData, JsonRequestBehavior.AllowGet);
    }
}

I also added a using block to properly dispose of the db object after use.