MVC5: Foreign Key and data access - asp.net-mvc

Question

I am looking for selecting a list from my table based on another table. I need to retrieve system names that are part of a particular system family. i have already added foreign keys. I created a ViewModel containing both these classes but it throws a null pointer exception. I am new to MVC and I am not sure where I am wrong.
Model Class : Systems
public class Systems
{
public int SystemsID { get; set; }
public string SystemName { get; set; }
public DateTime CreatedOn { get; set;}
public string CreatedBy { get; set; }
public int SystemFamilyID { get; set; }
public virtual SystemFamily SystemFamily { get; set; }
}
Class SystemFamily
public class SystemFamily
{
public int SystemFamilyID { get; set;}
public int SystemsID {get;set;}
public string FamilyName { get; set; }
public DateTime DateCreated { get; set; }
public string CreatedBy { get; set; }
public virtual ICollection<Systems> Systems { get; set; }
}
ViewSystem is a method in my SystemFamilyController.
public ActionResult ViewSystem(int? id)
{
var viewmodel = new Sys_SysFam();
ViewBag.SystemFamilyID = id.Value;
//if (id != null)
//{
// ViewBag.SystemFamilyID = id.Value;
// viewmodel.Systems = viewmodel.SystemFamily.Where(
// i => i.SystemFamilyID == id.Value).Single().Systems;
//}
return View(viewmodel);
}
the view :
#model SystemFam_System.ViewModel.Sys_SysFam
#{
ViewBag.Title = "ViewSystem";
}
<h2>ViewSystem</h2>
<p>#ViewBag.SystemFamilyID</p>
<table>
#foreach (var item in Model.Systems)
{
string selectedRow = "";
if (item.SystemFamilyID == ViewBag.SystemFamilyID)
{
//{
// selectedRow = "success";
//}
<tr class="#selectedRow">
<td>
#item.SystemName
</td>
<td>
#item.SystemsID
</td>
<td>
#item.SystemFamily
</td>
</tr>
}
}
</table>
I get null pointer Exception. I want to view the system that belongs to a particular family in view system.
Thanks in advance!!
Vini
Edit :
public class Sys_SysFam
{
public IEnumerable<Systems> Systems { get; set; }
public SystemFamily SystemFamily { get; set; }
}

Answer 1

Ok i have checked Sys_SysFam class too. As per your current code it will always throw null reference exception becasue in your controller code you are using:
public ActionResult ViewSystem(int? id)
{
var viewmodel = new Sys_SysFam();
ViewBag.SystemFamilyID = id.Value;
//if (id != null)
//{
// ViewBag.SystemFamilyID = id.Value;
// viewmodel.Systems = viewmodel.SystemFamily.Where(
// i => i.SystemFamilyID == id.Value).Single().Systems;
//}
return View(viewmodel);
}
here you are creating an object of Sys_SysFam as viewmodel and as your if part is commented so you are returning same viewmodel in which viewmodel.Systems will always be null. Here i did not see any request to database for getting the data from db but i think your data in viewmodel will come from database and if i uncomment your if condition then too you are not sending any request to database you are using same viewmodel object created above.
viewmodel.Systems = viewmodel.SystemFamily.Where(
i => i.SystemFamilyID == id.Value).Single().Systems;
in right side you are using viewmodel.SystemFamily with where condition but as viewmodel.SystemFamily is null it will always throw exception. Your solution should be something like this:
public ActionResult ViewSystem(int? id)
{
DataContext context = new DataContext();
var viewmodel = new Sys_SysFam();
ViewBag.SystemFamilyID = id.Value;
if (id != null)
{
ViewBag.SystemFamilyID = id.Value;
var sysFamily = context.SystemFamily.Include(x => x.Systems).FirstOrDefault(x => x.SystemFamilyID == id.Value);
if (sysFamily != null)
{
viewmodel.Systems = sysFamily.Systems;
}
}
return View(viewmodel);
}
here first i am creating object of DataContext which is my main context to access the database using entity framework. so first i will get the system family based on passed id from database and if system family is not null then i will set the data of systems in viewmodel. Include method will bring data for Systems based on system family from database.
Also improve your Sys_SysFam class to initialize systems so that it will not throw exception in your view when there is no data in viewmodel.Systems like this:
public class Sys_SysFam
{
public Sys_SysFam()
{
this.Systems = new List<Systems>();
}
public SystemFamily SystemFamily { get; set; }
public IEnumerable<Systems> Systems { get; set; }
}
Hope this will help you.

Answer 2

Remove SystemsID property from SystemFamily class because it is not used for ICollection virtual property. so your SystemFamily class should be like this:
public class SystemFamily
{
public int SystemFamilyID { get; set;}
public string FamilyName { get; set; }
public DateTime DateCreated { get; set; }
public string CreatedBy { get; set; }
public virtual ICollection<Systems> Systems { get; set; }
}

Answer 3

A friend of mine could find me a way. But it doesnt use any ViewModel. I would like to know how it need to be done with ViewModel as well..
public ActionResult ViewSystem(int? id)
{
var model = from item in db.Systems
orderby item.SystemsID
where item.SystemFamilyID == id
select item;
return View(model);
}